Reactivity of Cyclic Silenolates Revisited

Article abstract of DOI:10.1021/acs.organomet.7b00540

The stable exocyclic silenolates 2a-c (2a, R = Mes; 2b, R = o-Tol; 2c, R = 1-Ad) were fully characterized by NMR and UV-vis spectroscopy. According to spectroscopic and structural features, 2a-c are best described as acyl silyl anions (tautomeric structure I) in solution. This behavior is also reflected by the reaction of 2a,c with MeI. Both alkylation reactions take place at the corresponding silicon atom and lead to the formation of the methylated structures 4a,b in nearly quantitative yields. Furthermore, the thermal stability of exocyclic silenolates 2a,c was investigated. In the case of 2a, a thermally induced intramolecular sila-Peterson alkenation was observed at 60 °C. This transformation allowed straightforward access to 2-oxahexasilabicyclo[3.2.1]octan-8-ide 5 as a structurally complex, bicyclic silicon framework. In contrast to that, heating of 2c, as an example of an alkyl-substituted silenolate, led to an unexpected degradation to uncharacterized polymers. However, we were able to isolate the 1-adamantyl-substituted, bicyclic compound 8, which is structurally closely related to 5, by the treatment of 1,4-dipotassium-1,4-bis(trimethylsilyl)cyclohexasilane with 1 equiv of 1-adamantoyl chloride. Again an intramolecular sila-Peterson alkenation is responsible for the formation of 8. The mechanism for this highly selective reaction sequence is outlined and supported by density functional theory (DFT) calculations, which highlight the thermodynamic driving force and the low activation barriers of this multistep transformation.

Full text of DOI:10.1021/acs.organomet.7b00540

Article
pubs.acs.org/Organometallics
Reactivity of Cyclic Silenolates Revisited
Michael Haas,* Mario Leypold,* Lukas Schuh, Roland Fischer, Ana Torvisco, and Harald Stueger
Institute of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria
S
* Supporting Information
ABSTRACT: The stable exocyclic silenolates 2a−c (2a, R =
Mes; 2b, R = o-Tol; 2c, R = 1-Ad) were fully characterized by
NMR and UV−vis spectroscopy. According to spectroscopic
and structural features, 2a−c are best described as acyl silyl
anions (tautomeric structure I) in solution. This behavior is
also reflected by the reaction of 2a,c with MeI. Both alkylation
reactions take place at the corresponding silicon atom and lead
to the formation of the methylated structures 4a,b in nearly
quantitative yields. Furthermore, the thermal stability of
exocyclic silenolates 2a,c was investigated. In the case of 2a,
a thermally induced intramolecular sila-Peterson alkenation was
observed at 60 °C. This transformation allowed straightforward access to 2-oxahexasilabicyclo[3.2.1]octan-8-ide 5 as a
structurally complex, bicyclic silicon framework. In contrast to that, heating of 2c, as an example of an alkyl-substituted silenolate,
led to an unexpected degradation to uncharacterized polymers. However, we were able to isolate the 1-adamantyl-substituted,
bicyclic compound 8, which is structurally closely related to 5, by the treatment of 1,4-dipotassium-1,4-bis(trimethylsilyl)-
cyclohexasilane with 1 equiv of 1-adamantoyl chloride. Again an intramolecular sila-Peterson alkenation is responsible for the
formation of 8. The mechanism for this highly selective reaction sequence is outlined and supported by density functional theory
(DFT) calculations, which highlight the thermodynamic driving force and the low activation barriers of this multistep
transformation.
thermal stability of Li silenolates prevented them from using
this substance class for further derivatizations. Significant
improvement was achieved by Ottosson et al. in replacing
lithium with potassium as counterion.2g Potassium silenolates
were found to be thermodynamically more stable and thus
could be stored under an inert atmosphere and ambient
temperature over a few months without degradation. These
observations allowed Ottosson et al. some structural insights
into K silenolates. They found that the keto form is the
predominant tautomeric structure in K silenolates, which
manifests as a strongly pyramidal central silicon atom with an
elongated Si−C single bond.2g Further studies of Bravo-
Zhivotovskii and Apeloig et al. disclosed that the polarity of the
solvent used influences the stability of silenolates enormously.
In addition to this, they reported on the first synthesis,
isolation, and X-ray molecular structures of two enol form
silenolates, [(tBuMe2Si)2SiC(OLi)Ad and (tBu2MeSi)2Si
C(OLi)Ad], by taking advantage of nonpolar solvent systems.
These compounds display remarkably short Si−C bonds
(between 1.819 and 1.823 Å) and consequently adopt a planar
structure around the central silicon atom.2h
INTRODUCTION
■
The synthesis and characterization of silenolates is still a
challenging endeavor. As in the case of metal enolates, two
tautomeric structures for silenolates can be drawn: in the keto
form the negative charge resides predominantly on the silicon
atom (I), while in the enol form (II) the negative charge is
primarily located on the oxygen atom (Scheme 1).1
Scheme 1. Tautomeric Structures for Silenolates
The dominant structure of metal enolates is in general the
enol form and preferably exists in the solid state as well as in
solution.1 However, silenolates show in particular a significantly
different tautomeric behavior. The position of this equilibrium
is strongly influenced by the chosen alkali metal and the solvent
system as well as the substituent at the carbonyl moiety.2
Lithium silenolates were explored and reported first in the early
1990s. In this context, four scientists made valuable
contributions in this field, namely Bravo-Zhivotovskii,3
Apeloig,4 Ishikawa, and Ohshita.2a−e Unfortunately, the low
Later on, our group demonstrated the possibility of
synthesizing and characterizing cyclic silenolates. Furthermore,
the reaction of 2a−c with different chlorosilanes R3SiCl was
i
investigated. The reaction of Pr3SiCl with aryl-substituted
Received: July 18, 2017
© XXXX American Chemical Society
A
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silenolates 2a,b allowed the formation of previously unknown
silenes with exocyclic structures 3a,b.2i
We have recently reported on a series of stable exocyclic
germenolates.5 In contrast to germenolates, the structural
features of exocyclic silenolates seem to be strongly influenced
by the nature of the substituents attached to the carbonyl
moiety (see Figure 1). Exocyclic germenolates do not show
RESULTS AND DISCUSSION
■
UV−vis Spectroscopy and TDDFT-PCM Calculations.
The potassium silenolates 2a−c were synthesized according to
the same method as previously reported by our group.2i THF
and Et2O were used as solvents to determine the charge
transfer behavior for the longest wavelength absorption band.6
Figure 2 depicts the measured UV−vis spectra in THF and
Et2O together with their calculated frontier Kohn−Sham
orbitals. In order to examine the differences between aromatic
and saturated substituents at the carbonyl moiety, the mesityl-
and adamantyl-substituted derivatives 2a,c were investigated.
The longest wavelength absorption bands were assigned to two
conformational minimum structures due to the small energy
differences in cyclohexasilane conformers, which is caused by
the high flexibility of the polysilane framework.7 All UV−vis
calculations were performed via TDDFT-PCM in THF as well
as in Et2O at the B3LYP/6-31+G(d,p) level of theory. The
obtained experimental and computational data are summarized
in Table 1 and show reasonable agreement. Silenolates 2a,c
both exhibit an intense absorption maximum in the region
between 433 and 448 nm, which is red-shifted in the order 2c
→ 2a. The HOMOs of 2a,c (Figure 2) correspond mainly to
the pz orbital of the silicon atom with little variation in shape
and energy. Upon excitation, electron density is displaced into
the π* orbital of the carbonyl moiety (LUMO or LUMO+1).
In the LUMO of the aryl-substituted species 2a, our
calculations additionally showed considerable conjugation of
the carbonyl group and the aromatic π systems, which is not
possible for the alkyl-substituted silenolate 2c. As a
consequence of this, the empty orbitals (LUMO or LUMO
+1) are energetically stabilized in the order 2c → 2a. This
stabilization results in smaller excitation energies and in the
observed bathochromic shift of the corresponding absorption
bands. Therefore, silenolates 2a−c are best described as acyl
silyl anions (keto form in Scheme 1) in solution irrespective of
the nature of the R group attached to the carbonyl moiety. This
is of particular interest, because the reaction site of these
silenolates 2a−c with chlorosilanes as electrophiles was found
to be different. While 2c reacts with an equimolar amount of
iPr3SiCl at 0 °C in THF to give the Si-silylated product 1d, the
aromatic compounds 2a,b exclusively afforded the O-silylated
silenes 3a,b under the same conditions (Scheme 2). Although
calculated charges q(E1) from the natural population analysis
(NPA) at the PCM(THF) B3LYP/6-31+G(d,p) level revealed
a slightly higher negative charge placed on the central silicon
atom Si1 in 2c (−0.16) in comparison to 2a,b (−0.12, −0.13),
simple steric effects might be responsible for the observed
regioselectivity.2b The same tendency was also found in the
case of exocyclic germenolates.5
Figure 1. ORTEP diagram for compound 2c (above) and 2b (1:1
adduct with 18-crown-6). Thermal ellipsoids are depicted at the 50%
probability level. Hydrogen atoms are omitted for clarity.
similar substituent dependencies. On the basis of the observed
structural features (NMR data and DFT calculations), they are
best described as acyl germyl anions (keto form) with Ge−C
single bonds, CO double bonds, and markedly pyramidal
central Ge atoms. Interestingly, the reactivity of these
germenolates versus chlorosilanes parallels that observed earlier
for silenolates as described by Ohshita, Ishikawa, and our
group.2b,i
Due to the fact that our cyclic silenolates exhibit the same
reactivity in comparison to the published germenolates, we
investigated their electronic nature and chemical behavior in
more detail. Furthermore, the thermal stability of 2a,c was
explored and an anionic rearrangement cascade, specifically an
intramolecular sila-Peterson alkenation followed by the
trapping of the formed silene with an anionic oxygen
nucleophile, was found to occur under the given reaction
conditions. This allowed direct access to 2-
oxahexasilabicyclo[3.2.1]octan-8-ides as structurally complex
silicon frameworks. Moreover, we report on the UV−vis
spectroscopy of the cyclic silenolates 2a,c and their assignment
according to TDDFT-PCM calculations in THF and Et2O at
the B3LYP/6-31+G(d,p) level of theory. Finally, we focus on
the reactivity of 2a,c toward MeI.
In addition to this, a different coordination of the potassium
ion in the X-ray crystal structure of 2b (see Figure 1) was
found, which we attribute to crystal-packing effects. This
changed packing mode of the potassium ion in 2b can be
explained by the well-known interaction of the potassium ions
with the aryl groups.8 Moreover, this different coordination
causes an altered ring conformation of the cyclohexasilane ring
(twisted boat).
Reactivity of 2a,c toward MeI. In the reaction of 2a,c
with MeI, same reactivities in terms of reaction sites were
found. In both cases, alkylation of the silicon atom was
observed in nearly quantitative yields (Scheme 3). This
observation can be based on the classical orbital control,
B
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Figure 2. Measured UV−vis spectra and the calculated frontier Kohn−Sham orbitals in THF and Et2O (1 × 10−3 mol/L) at the TDDFT-PCM
B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory.
Table 1. Experimental and TDDFT-PCM B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) Calculated Absorption Bands λ in THF
and Et2O, Extinction Coefficients ε, and Oscillator Strengths f for the K Silenolates 2a,c
assignment
solvent
THF
λmax,exp (nm) (ϵ (L mol−1 cm−1))
conformation
λmax,calc (nm) (f)
2a
2c
448 (2576)
chair (60%)
boat (40%)
chair (60%)
boat (40%)
chair (61%)
429 (0.1210)
440 (0.1369)
434 (0.1124)
445 (0.1264)
418 (0.0640)
pz → π*(CO/aryl)
pz → π*(CO/aryl)
pz → π*(CO)
HOMO → LUMO
HOMO → LUMO
Et2O
THF
445 (2393)
438 (2571)
HOMO → LUMO
HOMO → LUMO+1
boat (39%)
chair (61%)
430 (0.0875)
422 (0.0622)
Et2O
433 (2447)
pz → π*(CO)
HOMO → LUMO
HOMO → LUMO+1
boat (39%)
435 (0.0869)
given the fact that the HOMOs of 2a,c are localized on the
silicon atom of the corresponding silenolate. The same
tendency was published in the case of acyclic silenolates by
the groups of Ohshita and Ottosson earlier.2b,g The methylated
silicon atom undergoes a significant low-field shift from −70 to
−45 ppm, which is caused by the lower shielding of the methyl
group in comparison to the trimethylsilyl group (see the
Experimental Section).
Thermal Behavior and Rearrangement Cascade of
Silenolates 2a,c. Interestingly, the yield of silene 3a was
drastically decreased when solutions of 2a were stored at room
temperature for several hours prior to quenching. This
observation encouraged us to investigate the stability of these
silenolates in more detail. We immediately discovered that a
new set of additional, clearly resolved signals grew in the 29Si
NMR spectra when toluene solutions of 2a were stirred at 22
°C for several hours. Prolonged reaction times (usually more
than 48 h) led to the formation of increasing amounts of
further degradation products of unknown structure. Raising the
temperature to 60 °C, however, not only resulted in an
accelerated product formation but also enhanced the selectivity
of this reaction. Thus, after a toluene solution of 2a was stirred
for 5 h at 60 °C, the 29Si NMR spectrum of the resulting
solution (compare Figure 3) was consistent with the formation
of one major rearrangement product, namely the structurally
C
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Scheme 2. Synthesis of Cyclic Silenolates 2a−c and Their
Scheme 4. Rearrangement Cascade Leading to the Mesityl
Substituted Carbanion 5 and Its Subsequent Conversion
with MeI to 6
i
Reactivity toward Pr3SiCl
will be trapped by the present oxygen nucleophile intra-
molecularly. It is worth noting that this general type of sila-
Peterson alkenation was simultaneously introduced by Oehme,
Apeloig, and Ishikawa.9 Kira and Scheschkewitz likewise
exploited this method in order to obtain silenes in a step-
economical way.10 A closely related pattern (first sila-aldol
reaction) was previously found by us when 1,4-bis(mesitoyl)
cyclohexasilane was reacted with equimolar amounts of KOtBu.
Again this silenolate underwent a rearrangement to the same
bicyclic structure.11
Scheme 3. Reactivity of 2a,c toward MeI
The 29Si NMR spectrum of the resulting solution together
with the spectrum of 2a is depicted in Figure 3. In addition to
the resonance lines arising from residual starting material and
minor byproducts, this spectrum showed eight signals
consistent with the presence of eight magnetically non-
equivalent silicon atoms. Previous experiences with structurally
related compounds allowed a qualitative assignment of the
spectrum. The signal at −71.1 ppm is assigned to the
bridgehead Si2 atom. The measured chemical shift value
compares reasonably well with those observed for structurally
related bicyclo[2.2.2]octasilanes.12 Furthermore, the incorpo-
ration of the oxygen atom into the cyclopolysilane framework
breaks the symmetry of the ring. Therefore, 5 displays three
SiMe2 signals in the typical range between −47 and −53 ppm
as well as two resonance lines for the SiMe3 groups at −12.2
and −17.3 ppm. The signal of the second bridgehead silicon
(Si5) and the endocyclic OSiMe2 group appear at a significantly
lower field (δ(29Si) −21.1 and 2.5 ppm, respectively) due to the
effect of the attached oxygen atom.13
highly interesting carbanion 2-oxahexasilabicyclo[3.2.1]octan-8-
ide 5.
This observation obviously implicated that the silenolate 2a
was only the kinetic product, which thermodynamically
rearranged via a mild, selective silyl-migration cascade to the
bicyclic carbanion 5 (Scheme 4). Representing the first example
of an intramolecular sila-Peterson alkenation, the formed silene
Unfortunately, all attempts to crystallize 5 failed. Moreover,
stored reaction solutions of 5 afforded increasing amounts of
unidentified decomposition products. Concentration of the
reaction solution in vacuo to facilitate crystallization also caused
extensive product degradation. However, a definite proof for
the proposed structure of 5 was finally obtained after
derivatization. Upon the addition of MeI to a freshly prepared
toluene solution of 5, the corresponding methylated bicyclic
adduct 6 was formed in the diastereospecific ratio endo:exo =
2:1. The resulting isomeric mixture was subsequently isolated
by preparative thin-layer chromatography. Analytical and
spectroscopic data (for details consult the Experimental
Section) clearly support the bicyclic structure of 6 and 5.
The endo diastereomer of 6, furthermore, could be crystallized
from acetone and was subjected to X-ray diffraction analysis.
Figure 3. 29Si NMR spectroscopy (INEPT, bb decoupled) of a toluene
solution of 2a before (top) and after thermolysis for 5 h at 60 °C
(bottom).
D
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This resulting molecular structure is depicted in Figure 4 along
with selected bond distances and bond and dihedral angles.
Scheme 5. Reaction Cascade Leading to the Formation of
the Adamantyl-Substituted Carbanion 7 and its H-
Abstraction Product 8
Figure 4. ORTEP diagram for compound (1R,5R,8R)-endo-6 (endo
configuration). Thermal ellipsoids are depicted at the 50% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and bond angles (deg) with estimated standard deviations: Si(1)−
O(1) 1.661(1), Si(2)−O(1) 1.651(1), Si(1)−C(1) 1.953(1), Si(4)−
C(1) 1.980(1), Si−Si (mean) 2.372; Si(1)−O(1)−Si(2) 141.0(1),
Si(1)−Si(6)−Si(7) 98.1(1), Si(6)−Si(7)−Si(4) 99.8(1), Si(7)−
Si(4)−Si(3) 107.7(1), Si(4)−Si(3)−Si(2) 104.8(1).
endo-6 crystallized in the monoclinic space group P21/c with
unexceptional bond lengths and angles. The unit cell contains
four molecules as a racemic mixture (see the Supporting
Information).
Density functional theory (DFT) calculations were per-
formed for the observed rearrangement reactions of mono-
acylcyclohexasilanyl anions to elucidate the reaction mechanism
responsible for the highly selective formation of the bicyclic
structures 5 and 7. To reduce the complexity of the simulated
systems, we chose acetyl and benzoyl groups instead of the
adamantoyl and the mesitoyl moieties together with a
hydrosilane skeletal framework. The resulting model systems
are depicted in Chart 1. Computational details can be found in
the Experimental Section.
After a toluene solution of 2c (R = Ad), as an example of an
alkyl-substituted silenolate, was stirred for 3 h at 60 °C,
significant polymerization of the starting material was detected.
Prolonged warming of the reaction solution led to complete
degradation of 2c to uncharacterized material. Therefore, we
assumed that, in the case of alkyl-substituted systems, the
negative charge could not be distributed in the same way as in
compound 5 and the primarily formed carbanion 7 reacted
further under the applied reaction conditions (compare NPA
charges in Computational Studies for aromatic and aliphatic
carbanions: NPA charge (Bz-J) = −1.33 vs NPA charge (Ac-J)
= −1.40).
Chart 1. Model Substances for DFT Calculations
It is quite noteworthy that the 1-adamantyl-substituted
bicyclic compound 8, which is structurally closely related to 5,
was obtained as the final product upon treating 1,4-
dipotassium-1,4-bis(trimethylsilyl)cyclohexasilane with 1 equiv
of 1-adamantoyl chloride (AdCOCl) and stirring the resulting
solution for 30 min at 22 °C (Scheme 5). In accordance with
our computational studies presented below, the formation of 8
can be rationalized by assuming the intermediate formation of
the silanide 9, which subsequently rearranges to give the
bicyclic carbanion 7. Apparently, 7 is very unstable, losing its
intense red color within minutes presumably by the abstraction
of one proton from the surrounding media to give 8 as the final
product. 8 was isolated from the reaction mixture after
extractive workup and preparative thin-layer chromatography
(prep-TLC) as a colorless solid in 69% yield. The structure
proposed for 8 is well established by the analytical data
summarized in the Experimental Section and by the results of
single-crystal X-ray diffraction studies (see the Supporting
Information). It is worth mentioning that the same outcome
was observed when 2 equiv of 1-adamantoyl chloride was
reacted with the dianion. In this case, the primary aim was to
synthesize the corresponding bisacylsilane.
The computed reaction profiles for the rearrangement of Bz-
A and Ac-E are depicted in Figure 5. Our calculations
demonstrate that the rearrangement of the alkyl- and aryl-
substituted monoacyl anions follow identical reaction pathways
indicated by similar activation energies for equal trans-
formations with values consistently lower than ΔΔG° = 20.0
kcal mol−1. However, as shown in Scheme 5, alkyl-substituted
cyclic silenolates are synthetically not accessible. The reaction
cascade, therefore, starts directly from the anion Ac-E in that
case.
Scheme 6 presents the resulting overall reaction steps leading
to the formation of the phenyl- and methyl-substituted bicyclic
final products Bz-J and Ac-J, respectively, representing
structures 5 and 7. For Bz-J, the most stable conformer Bz-A
of the silenolate needs to rearrange into the boat conformation
Bz-B for the initial 1,4-silyl migration with a calculated
E
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Figure 5. Computed reaction profile for the rearrangement cascade of Bz-A and Ac-E calculated at the PCM(THF) B3LYP/6-311+G(2df,p)//
B3LYP/6-31+G(d) level of theory. Relative free energy values ΔΔG° are given in relation to Bz-A and Ac-E, respectively, as starting points for the
rearrangement cascade. Hydrogen atoms are omitted for better clarity in the depicted structures.
a
Scheme 6. Computed Rearrangement Cascade for the Formation of Aryl-Substituted 2-Oxahexasilabicyclo[3.2.1]octane-8-ides
a
Relative free activation barriers ΔΔGAE° are shown for the calculated benzoyl derivative Bz-A at the PCM(THF) B3LYP/6-311+G(2df,p)//
B3LYP/6-31+G(d) level of theory.
activation energy of ΔΔG° = 25.8 kcal mol−1. Under formal
Walden inversion of the configuration at the migrating silicon
atom, this transformation is comparable to the well-studied
Brook-type rearrangements in the literature and presumably
proceeds intramolecularly.14 The calculated value of ΔΔG° =
13.8 kcal mol−1 for the subsequent pyramidal silyl anion
inversion of Bz-C to Bz-D through the planar transition state
TSBz‑C‑D is consistent with calculated inversion barriers of silyl
anions, extensively studied by Flock and Marschner in 2002.15
In this context, three silyl substituents in position α to the
negatively charged silicon atom of compound Bz-C are known
to stabilize the negative charge via hyperconjugation and lower
the conformational stability of this intermediate.15 After
conformational isomerization of Bz-D to the rotamer Bz-E,
intramolecular, nucleophilic attack of the silyl anion on the
carbonyl moiety in Bz-E occurs with a relatively low activation
barrier of ΔΔG° = 12.2 kcal mol−1. This step provides a
straightforward access to the highly symmetric bicycle Bz-F, in
F
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spectrometer in C6D6 or CDCl3 solution and referenced versus
TMS using the internal H-lock signal of the solvent. The starting
which the negative charge becomes punctually localized on the
oxygen atom (NPA charge −0.92). This fact subsequently leads
to the breakup of the silicon−silicon framework of Bz-F and
the former carbonyl oxygen becomes incorporated into the ring
system, forming intermediate Bz-G (TSBz‑F‑G: ΔΔG° = 2.6 kcal
mol−1). A second pyramidal silyl anion inversion of Bz-G with
an inversion barrier of ΔΔG° = 14.1 kcal mol−1 affords
substructure Bz-H. In the next step stereospecific elimination to
the corresponding silene Bz-I again generates a highly
silicophilic alkoxide as a potential nucleophile (NPA charge
−1.33) with an activation barrier of ΔΔG° = 13.1 kcal mol−1.
This step completes the intramolecular sila-Peterson alkena-
tion. The final cyclization of the free alkoxide within silene Bz-I
terminates the proposed reaction sequence and gives the
exceptionally stable product Bz-J with an calculated overall
reaction energy of ΔΔG° = −41.4 kcal mol−1. Moreover, this
simulated reaction pathway emphasizes the need for warming
in the case of aromatic substituents for a more selective
reaction, because of the initial 1,4-silyl migration. This
transformation is not required in compounds with aliphatic
residues.
2
materials 1,1,4,4-tertakis(trimethylsilyl)octamethylcyclohexasilane and
1,4-dipotassium-1,4-bis(trimethylsilyl)octamethylcyclohexasilane were
synthesized according to published procedures.17 HRMS spectra were
run on a Kratos Profile mass spectrometer equipped with a solid probe
inlet. Infrared spectra were obtained on a Bruker Alpha-P Diamond
ATR Spectrometer from the solid sample. Melting points were
determined using a Buechi 535 apparatus and are uncorrected.
Elemental analyses were carried out on a Hanau Vario Elementar EL
apparatus. UV absorption spectra were recorded on a PerkinElmer
Lambda 5 spectrometer.
Synthesis of 4a. A 300 mg portion (0.458 mmol, 1.00 equiv) of 1a
was dissolved in 4 mL of THF and cooled to −78 °C, and 53.9 mg
(0.481 mmol, 1.05 equiv) of KOtBu was added. After it was stirred for
an additional 30 min, the mixture was warmed to room temperature
and finally stirred for an additional 60 min (the reaction mixture
turned dark red). At this time reaction control by NMR spectroscopy
showed clean conversion of the starting material to the desired
silenolate 2a, Me3SiOtBu, and Me3SiOSiMe3. Now the resulting red
solution was cooled to 0 °C and 0.2 mL of MeI was added dropwise.
The solution immediately turned yellow. After aqueous workup with
10 mL of 3% sulfuric acid the organic layer was separated and dried
over Na2SO4 and the solvents were stripped off on a rotary evaporator.
Drying under vacuum (0.001 mbar) and crystallization from acetone
solution at −30 °C afforded 190 mg (70%) of the analytically pure
acylcyclohexasilane 4a as yellow crystals.
CONCLUSION
■
In summary, we investigated the UV−vis spectroscopic
behavior of the cyclic silenolates 2a−c and could assign the
longest wavelength transitions according to TDDFT-PCM
calculations in THF and Et2O at the B3LYP/6-31+G(d,p) level
of theory. We can state that silenolates 2a−c are best described
as acyl silyl anions (keto form in Scheme 1) in solution,
irrespective of the residue attached to the carbonyl function.
This is also reflected in the reactivity of 2a,c toward MeI and
the thermal rearrangement of 2a, which represents to the best
of our knowledge the first example of an intramolecular sila-
Peterson alkenation followed by an intramolecular trapping
reaction of the bound alcoholate. These rearrangement
reactions gave rise to a highly selective route for the formation
of 2-oxahexasilabicyclo[3.2.1]octan-8-ides 5 and 7 as a
structurally complex silicon framework with aromatic as well
as aliphatic residues. Hereby, silyl anions with cyclic structures
were identified as global intermediates of this rearrangement
cascade that happens to occur under given reaction conditions.
These anionic species were fully characterized by NMR
spectroscopy and X-ray crystallography after derivatization
with MeI. Methylation of the monomesitylated anion 5
preferentially resulted in the regiospecific formation of the
corresponding endo stereoisomer endo-6 in a ratio of 2:1.
Density functional theory (DFT) calculations highlight the
thermodynamic driving force and the low activation barriers
during this rearrangement cascade with an overall reaction
energy of about ΔΔG° = −40.0 kcal mol−1. Since most of the
individual activation barriers are fairly small, these simulations
could perfectly explain why the reaction proceeds smoothly
with aliphatic substituents and why warming is essential for
compounds with aromatic moieties caused by an initial,
energetically unfavorable 1,4-silyl migration.
Mp: 120−121 °C. Anal. Found: C, 50.30; H, 9.25. Calcd for
C25H56OSi8: C, 50.26; H, 9.45. 29Si NMR (CDCl3, TMS, ppm):
−7.27, −8.82 (SiMe3); −37.33, −40.86 (SiMe2); −45.73 (SiCO);
−131.32 (Si(SiMe3)2). 13C NMR (CDCl3, TMS, ppm): 251.45 (C
O); 144.20, 137.75, 131.55, 128.70 (Mes-C); 21.03, 19.31 (Mes-CH3);
3.70, 3.61 (Si(CH3)3); −0.78, −1.72, −4.70, −5.27 (Si(CH3)2); −6.27
(SiCOMe). 1H NMR (CDCl3, TMS, ppm): 6.78 (s, 2H, Mes-H); 2.26
(s, 6H, Mes-CH3); 2.15 (s, 3H, Mes-CH3); 0.34−0.21 (s, 45H,
Si(CH3)2, Si(CH3)3 and SiCOCH3). IR (neat): ν(CO) 1610 (m)
cm−1. HRMS: calcd for [C25H56OSi8]•+ (M+), 596.2485; found,
596.2469.
Synthesis of 4b. The same procedure as for 4a was followed with
1.00 g (1.50 mmol, 1.00 equiv) of 1c and 176 mg (1.56 mmol, 1.04
equiv) of KOtBu. Yield: 568 mg (62%) of 4b as colorless crystals.
Mp: 166−169 °C. Anal. Found: C, 50.95; H, 9.83. Calcd for
C26H60OSi8: C, 50.91; H, 9.86. 29Si NMR (C6D6, TMS, ppm): −7.79,
−8.40 (SiMe3); −37.84, −41.62 (SiMe2); −45.45 (SiCO); −131.32
(Si(SiMe3)2). 13C NMR (C6D6, TMS, ppm): 245.74 (CO); 51.68
(Ad-C−CO); 36.68, 36.54 (Ad-CH2); 28.04 (Ad-CH); 3.54, 3.46
(Si(CH3)3); 1.03, 1.48, −4.21, −5.26 (Si(CH3)2); −3.24 (SiCOMe).
1H NMR (CDCl3, TMS, ppm): 2.05, 1.77, 1.70, 1.65 (15H, Ad-H);
0.54 (s, 3H, Si(CH3)); 0.24, 0.23 (s, 18H, Si(CH3)3); 0.36, 0.26, 0.18,
0.13 (s, 6H each, Si(CH3)2). IR (neat): ν(CO) 1612 (m) cm−1.
HRMS: calcd for [C26H60OSi8]•+ (M+), 612.2798; found, 612.2828.
Thermolysis of 2a. A 300 mg (0.458 mmol, 1.00 equiv) portion of
1a and 127 mg (0.481 mmol, 1.05 equiv) of 18-crown-6 were
dissolved in 4 mL of toluene and cooled to −78 °C. Subsequently,
53.9 mg (0.481 mmol, 1.05 equiv) of KOtBu was added and the
resulting mixture was stirred at this temperature for an additional 30
min. After this time, the solution was warmed to room temperature
and finally stirred until the starting material had been totally consumed
(monitored by 29Si NMR spectroscopy). Now the resulting solution of
2a was stirred at 60 °C for 17 h, after which time complete conversion
to the carbanionic species 5 was observed by 29Si NMR. Due to its
high reactivity the product could not be crystallized from the reaction
solution. Upon the attempted removal of the volatile components in
vacuo, 1a decomposed to uncharacterized degradation products.
29Si NMR (D2O, TMS, ppm): −2.49, −12.16 (SiMe3); −17.30,
−47.91, −48.24, −52.03 (SiMe2); −21.14 (Si-OSiMe2); −71.03 (Siq).
Synthesis of 6. A 500 mg (0.763 mmol, 1.00 equiv) portion of 1a
and 212 mg (0.801 mmol, 1.05 equiv) of 18-crown-6 were dissolved in
10 mL of toluene and cooled to −78 °C. Subsequently, 89.9 mg (0.801
mmol, 1.05 equiv) of KOtBu was added and the resulting mixture was
EXPERIMENTAL SECTION
■
General Considerations. All experiments were performed under a
nitrogen atmosphere using standard Schlenk techniques. Solvents were
dried using a column solvent purification system.16 Commercial
KOtBu (97%), MesCOCl (99%), and AdCOCl (98%), were used as
purchased. 1H (299.95 MHz), 13C (75.43 MHz), and 29Si (59.59
MHz) NMR spectra were recorded on a Varian INOVA 300
G
DOI: 10.1021/acs.organomet.7b00540
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
stirred at this temperature for an additional 30 min. After this time, the
solution was warmed to room temperature and finally stirred until the
starting material had been completely consumed (monitored by 29Si
NMR spectroscopy). After complete conversion the reaction solution
was heated to 60 °C. After the silenolate 2a was completely converted
to the carbanionic species 5 (monitored by 29Si NMR spectroscopy),
an excess of MeI was added to the reaction solution. The reaction
solution immediately turned colorless. After aqueous workup with 10
mL of 3% sulfuric acid, the organic layer was separated and dried over
Na2SO4. After removal of the volatile components on a rotary
evaporator, the product was chromatographed on a precoated TLC
plate SIL G-200 UV254, with heptane as eluent, to give 427 mg (94%)
of endo-6 and exo-6. Crystallization from acetone at −30 °C afforded
260 mg (57%) of the analytically pure endo-6 as colorless crystals.
Data for endo-6 are as follows. Mp: 177−179 °C. Anal. Found: C,
50.45; H, 9.21. Calcd for C25H56OSi8: C, 50.26; H, 9.45. 29Si NMR
(CDCl3, TMS, ppm): −7.54, −12.53 (SiMe3); −14.89 (Siq-OSiMe2);
7.31, −40.04, −42.07, −44.77 (SiMe2); −53.82 (Siq). 13C NMR
(CDCl3, TMS, ppm): 144.65, 138.14, 134.77, 133.11, 131.57, 129.83
(aryl-C); 28.54, 28.47, 26.64 (aryl-CH3); 24.64 (C-CH3); 20.20
(−CH3); 4.51 (Si(CH3)2); 4.10 (Si(CH3)3); 2.67 (Si(CH3)2); 0.36
(Si(CH3)3; −0.48 (Si(CH3)2; −1.33 (Si(CH3)2); −2.04 (Si(CH3)2);
imaginary frequency, respectively. Single-point energy calculations
were carried out on the B3LYP/6-31+G(d) optimized geometries at
the B3LYP/6-311+G(2df,p) level of theory. Solvent effects were
considered by using the self-consistent reaction field (SCRF) method
based on the polarizable continuum model (PCM) for tetrahydrofur-
ane (THF) and diethyl ether (Et2O) with dielectric constants ε(THF)
= 7.4257 and ε(Et2O) = 4.2400, respectively.23 Vertical excitations of
the conformational minimum structure of silenolates 2a,c were
calculated via time-dependent DFT (TDDFT) calculations at the
PCM(THF) or PCM(Et2O) TDDFT B3LYP/6-31+G(d,p)//B3LYP/
6-31+G(d,p) level of theory.20,21,23,24 The relative Gibbs free energies
(ΔΔG°) at 298.15 K and 1 atm are reported in kcal mol−1. Atomic
charges were obtained by natural population analyses (NPA)
calculations as implemented in the Gaussian09 program. All computed
structures were visualized using the Gabedit software package.25 For
more details see the Supporting Information.
X-ray Crystallography. All crystals suitable for single-crystal X-ray
diffractometry were removed from a vial and immediately covered with
a layer of silicone oil. A single crystal was selected, mounted on a glass
rod on a copper pin, and placed in the cold N2 stream provided by an
Oxford Cryosystems cryostream. XRD data collection was performed
for all compounds on a Bruker APEX II diffractometer with use of Mo
Kα radiation (λ = 0.71073 Å) and a CCD area detector. Empirical
absorption corrections were applied using SADABS.26,27 The
structures were solved with use of the intrinsic phasing option in
SHELXT and refined by the full-matrix least-squares procedures in
SHELXL.28−30 The space group assignments and structural solutions
were evaluated using PLATON.31,32 Non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were located in calculated
positions corresponding to standard bond lengths and angles. Whole
molecule disorder, as observed in compound 8, was handled by
modeling the occupancies of the individual orientations using free
variables to refine the respective occupancy of the affected fragments
(PART).33 Disordered positions were refined using 60/40 split
positions. The distances between arbitrary atom pairs were restrained
to possess the same value using the distance restraints (DFIX) to
certain target values. Anisotropic Uij values of some atoms were
restrained (ISOR) to behave more isotropically. Crystallographic data
(excluding structure factors) have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publications CCDC-
1561727 (4a), CCDC-1561726 (4b), CCDC-1529184 (6), and
CCDC-1529182 (8).
1
−2.09 (Si(CH3)2); −3.08 (Si(CH3)2); −5.07 (Si(CH3)2). H NMR
(CDCl3, TMS, ppm): 6.75 (s, 1H, aryl-H); 6.64 (s, 1H, aryl-H); 2.58
(s, 3H, aryl-CH3); 2.48 (s, 3H, aryl-CH3); 2.18 (s, 3H, aryl-CH3); 1.38
(s, 3H, C−CH3); 0.43 (s, 3H, Si(CH3)2); 0.41 (s, 3H, Si(CH3)2); 0.32
(s, 6H, Si(CH3)2); 0.29 (s, 3H, Si(CH3)2); 0.26 (s, 3H, Si(CH3)2);
0.15 (s, 12H, Si(CH3)2 and Si(CH3)3); 0.02 (s, 9H, Si(CH3)3); −0.06
(s, 3H, Si(CH3)2). HRMS: calcd for [C25H56OSi8]•+ (M+), 596.2485;
found, 596.2516.
Data for exo-6 are as follows. 29Si NMR (CDCl3, TMS, ppm):
−7.50, −15.74 (SiMe3); 5.25 (Siq-OSiMe2); 13.18, −30.54, −32.98,
−42.99 (SiMe2); −57.00 (Siq).
Synthesis of 8. A solution of 1,4-dipotassium-1,4-bis-
(trimethylsilyl)octamethylcyclohexasilane in 10 mL of DME was
freshly prepared from 500 mg (0.860 mmol, 1.00 equiv) of
(Me3Si)2Si6Me8(SiMe3)2 and 203 mg (1.81 mmol, 2.10 equiv) of
KOtBu. The solution was stirred until the starting material was
completely consumed (monitored by NMR spectroscopy). Afterward
the solvent was removed under reduced pressure and the resulting
brownish oil was redissolved in 10 mL of toluene and slowly added to
a solution of 179 mg (0.900 mmol, 1.05 equiv) of 1-adamantoyl
chloride in 20 mL of diethyl ether at −78 °C. Subsequently, the red
solution was stirred for another 30 min at −78 °C, warmed to room
temperature, and finally stirred for an additional 30 min. After
decolorization an aqueous workup with 100 mL of 3% sulfuric acid was
performed. The organic layer was separated and dried over Na2SO4
and the solvents were stripped off on a rotary evaporator. Finally, the
crude material was chromatographed on a precoated TLC-plate SIL G-
200 UV254, with heptane as eluent, to give 357 mg (69%) of 8.
Mp: 218−222 °C. Anal. Found: C, 50.17; H, 9.68% Calcd for
C25H58OSi8: C, 50.09; H, 9.75. 29Si NMR (C6D6, TMS, ppm): 8.57
(OSiMe2); 6.81 (Siq-OSiMe2); −7.36, −16.85 (SiMe3); −39.43,
−43.33, −48.13 (SiMe2); −79.34 (Siq). 13C NMR (C6D6, TMS,
ppm): 47.01, 37.53, 36.50, 29.49 (Ad-C); 35.34 (CH-Ad); 2.91, 0.32
(Si(CH3)3); 3.15, 2.55, −1.25, −2.27, −2.34, −2.70, −5.52, −6.55
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00540.
NMR spectra, X-ray data, and details of computations
(PDF)
Cartesian coordinates of calculated structures (XYZ)
Accession Codes
CCDC 1529182, 1529184, and 1561726−1561727 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
1
(Si(CH3)2). H NMR (CDCl3, TMS, ppm): 1.97−1.58 (m, 16H, Ad-
H and CH-Ad); 0.49, 0.42, 0.39, 0.36, 0.28, 0.26, 0.25, 0.17 (s, 3H
each, Si(CH3)2); 0.34, 0.32 (s, 9H each, Si(CH3)3). HRMS: calcd for
[C25H58OSi8]•+ (M+), 598.2642; found, 598.2646.
Computational Studies. All computational studies were executed
on a computing cluster with blade architecture using the Gaussian09
software package.18 Geometry optimizations of all structures (local
minima (LMs) and transition states (TSs)) were performed in the gas
phase with B3LYP as the hybrid density functional19,20 together with
the 6-31+G(d) or 6-31+G(d,p) split-valence basis set of Pople and co-
workers for all atoms.21 The connectivity of TS structures was
confirmed by intrinsic reaction coordinate (IRC) analyses at the same
computational level.22 All stationary points (LMs and TSs) were
characterized by harmonic frequency calculations, yielding none or one
AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.H.: michael.haas@tugraz.at.
*E-mail for M.L.: mario.leypold@tugraz.at.
■
ORCID
Michael Haas: 0000-0002-9213-940X
H
DOI: 10.1021/acs.organomet.7b00540
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(13) Compare the value of δ(29Si) 8.32 ppm for the OSiMe2 groups
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Kovacs, I. J. Organomet. Chem. 1992, 431, 1−15.
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Soc. 1971, 93, 6221−6223. (c) Brook, A. G.; Pascoe, J. D. J. Am. Chem.
Soc. 1971, 93, 6224−6227.
(15) Flock, M.; Marschner, C. Chem. - Eur. J. 2002, 8, 1024−1030.
(16) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518−1520.
(17) Fischer, R.; Konopa, T.; Ully, S.; Baumgartner, J.; Marschner, C.
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(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, N.; Keith, T. V.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian 09,
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Harald Stueger: 0000-0001-8531-1964
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge support from NAWI Graz.
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