Stereocontrolled Syntheses of Functionalized cis- And trans-Siladecalins

Article abstract of DOI:10.1021/jacs.9b09902

We report the synthesis of both diastereomers of an all-silicon analog of decalin. Carbocyclic decalin is a ubiquitous bicyclic structural motif. The siladecalin synthesis provides materials functionalized with either Si-Ph or Si-H groups, versatile entry points for further chemical diversification. The synthesis of silicon-stereogenic silanes is significantly less precedented than the synthesis of asymmetric carbon centers, and strategies for control of relative stereochemistry in oligosilanes are hardly described. This study offers insights of potential generality, such as the epimerization of the cis-isomer to the thermodynamically downhill trans-isomer via a hypothesized pentavalent intermediate. Decalin is a classic example in the conformational analysis of organic ring systems, and the carbocyclic diastereomers have highly divergent conformational profiles. Like the carbocycle, we observe different conformational properties in cis- and trans-siladecalins with consequences for NMR spectroscopy, optical properties, and vibrational spectroscopy. This study showcases the utility of targeted synthesis for preparing complex and functionalized polycyclic silanes.

Full text of DOI:10.1021/jacs.9b09902

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Article
Stereocontrolled syntheses of functionalized cis- and trans-siladecalins
Eric A. Marro, Carlton P. Folster, Eric M. Press, Hoyeon Im,
John T. Ferguson, Maxime A. Siegler, and Rebekka S. Klausen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09902 • Publication Date (Web): 10 Oct 2019
Downloaded from pubs.acs.org on October 12, 2019
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Stereocontrolled syntheses of functionalized cis- and trans-
siladecalins.
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Eric A. Marro, Carlton P. Folster, Eric M. Press, Hoyeon Im, John T. Ferguson, Maxime A. Siegler,
Rebekka S. Klausen*
Department of Chemistry, Johns Hopkins University, 3400 N. Charles St, Baltimore, MD, 21218.
ABSTRACT: We report the synthesis of both diastereomers of an all-silicon analog of decalin, the ubiquitous bicyclic structural
motif. The synthesis provides materials functionalized with either Si–Ph or Si–H groups, versatile entry points for further chemical
diversification. The synthesis of silicon-stereogenic silanes is significantly less precedented than the synthesis of asymmetric carbon
centers and strategies for control of relative stereochemistry in oligosilanes are hardly described. This study offers insights of potential
generality, such as the epimerization of the cis-isomer to the thermodynamically downhill trans-isomer via a hypothesized pentavalent
intermediate. Decalin is a classic example in the conformational analysis of organic ring systems and the carbocyclic diastereomers
have highly divergent conformational profiles. Like the carbocycle, we observe different conformational properties in cis- and trans-
siladecalins with consequences for NMR spectroscopy, optical properties, and vibrational spectroscopy. This study showcases the
utility of targeted synthesis for preparing complex and functionalized polycyclic silanes.
Introduction
multiple
products
of
dimethyldichlorosilane
and
trimethylchlorosilane reductive coupling.20 The major products
were a polysilane (78%) and dodecamethylcyclohexasilane
(Si6Me12, 10%). Six other silanes were isolated and one with the
molecular formula of Si10Me18 (4% yield) was suggested to be
permethylsiladecalin. However, definitive assignment was not
possible until Hengge later identified this product as the trans-
siladecalin by X-ray crystallography.21
Silicon, a central material in technology and energy science,
possesses a diamond lattice crystal structure that inspires
interest in the properties of complex polycyclic silanes.1
However, polycyclic silanes are a significant challenge for
target-oriented
synthesis.2–6
Silicon
clusters
(e.g.
octasilacubane7,8) are typically more oxidatively sensitive than
the corresponding hydrocarbons.9 These Group 14 platonic
solids have been prepared by reductive coupling of halosilane
precursors, although reductive coupling limits both scale and
molecular diversity: low selectivity for a target compound, with
correspondingly low yields, is common and few functional
groups tolerate the strongly reducing reaction conditions.
We describe the synthesis of functionalized siladecalin
scaffolds via disilanide-halosilane coupling. Two advantages
accrue from this approach: control of relative stereochemistry
and functional group interconversion.
The preparation of silicon-stereogenic silanes has focused on
monosilanes as reagents and intermediates in asymmetric
organic synthesis,22–27 with isolated examples of cyclohexa- or
cyclopentasilanes exhibiting cis-trans stereoisomerism.13,28 We
include comprehensive details of our efforts to control relative
stereochemistry at the siladecalin ring fusion. Ultimately, we
identified a selective route to the cis-siladecalin skeleton, which
could be equilibrated to the lower energy trans-isomer.
Nonetheless, polycyclic silanes exhibit unusual properties
that are less well-understood than linear silanes.10,11 In a recent
example, Kyushin synthesized the bowl-shaped tricyclic silane
1 in 5% yield via reduction of a hexabrominated decasilane.12
The molecule’s bowl shape induces overlap of the Si-C *
orbitals such that the LUMO has “pseudo-π*” symmetry,
resulting in a bathochromically shifted absorption spectrum
relative to linear oligosilanes of a similar size.
The selective synthesis of both diastereomers enables the
description of isomer-dependent properties. The diastereomers
of carbocyclic decalin have markedly different properties.29 The
trans-decalin structure does not allow for ring inversion (chair
flipping) and the room temperature 1H NMR spectrum shows a
broad, partially resolved band composed of overlapping axial
and equatorial protons.30,31 In contrast, chair interconversion in
cis-decalin is rapid, resulting in a single sharp resonance even
at –121 ºC.30 Similar conformational dynamics in siladecalin
scaffolds underly differences in optical properties described
herein.
Silyl anions are a class of important intermediates for the
synthesis of cyclosilanes. Marschner described the synthesis of
bridged cyclosilanes, such as 2 and silanorbornane 3,13,14 from
the coupling of potassiosilyl anions15 (potassium silanides) with
halosilanes. Lewis acid-mediated skeletal rearrangement of a
derivative of 3 led to the landmark synthesis of siladamantane
4.16 Silanide chemistry17 has also made an impact in the rare
preparation of functionalized polycyclic silanes. Nuckolls et al.
converted 2 into thiomethyl-functionalized 6, as part of a study
demonstrating conformational control of σ-conjugation and
molecular conductance.18,19
Our synthesis opens the door to the preparation of a wide
variety of functionalized siladecalin skeletons. The diaryl
disilanide used in this study enables site-specific
Siladecalin is a long-standing challenge for chemical
synthesis. In 1972, West reported the characterization of the
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functionalization. Arylsilanes are latent electrophiles: treatment
into the siladecalin. The Si–H bond is a versatile functional
group for further derivatization via hydrosilation and other
chemistries.33–35
with strong acid (e.g. triflic acid)32 reveals a reactive silyl
triflate with formation of benzene. Here, we show that a
dearylation/hydrogenation sequence introduces Si–H residues
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Figure 1. Polycyclic silanes. a) Examples of fused polycyclic methylsilanes synthesized by reductive coupling. b) Examples of bridged
polycyclic methylsilanes synthesized by salt metathesis. c) Functionalized polycyclic silanes. d) Functionalized siladecalin retrosynthesis
and avenues of stereochemical control.
Results and Discussion
was reported to proceed with inversion of stereochemistry.43
We also anticipated the possibility of thermodynamic
equilibration of the cis- and trans-siladecalins.
1. Synthesis. We envisioned a different approach to
siladecalin synthesis via coupling of a disilanide and a
bifunctional cyclosilane (Figure 1d). Siladecalin Si10Ph4 could
Scheme 1. Synthesis of cyclohexasilane 13. i) iPrMgCl, THF,
0 °C  rt, 15 h, 73% ii) 1.) TfOH (2.0 equiv.), DCM, 0 °C
 rt, 2 h / LiBr, Et2O, r.t., 98% iii) Mg (Powdered, 2.5
equiv.), THF, reflux, 2.5 h, 54%.
arise from coupling of
7 and previously unreported
dichlorocyclohexasilane 8. Earlier work from our group
identified disilanide [(18-cr-6)K]+ 7 as a versatile intermediate
2
in the preparation of site-selectively functionalized silicon
rings.36–39 We further expected that Si10Ph4 would serve as an
entry point to a family of siladecalin frameworks, such as
Si10H4, via acid-mediated dearylation and derivitization.32
This synthetic approach provides at least two avenues for
control of the relative stereochemistry at the siladecalin ring
fusion. Cyclohexasilane 8 is itself chiral – could the building
block dictate the outcome of the coupling reaction? The
stereochemical course of nucleophilic substitution at tetrahedral
silicon atoms has long been recognized to be more complex
than for alkanes and examples of both retention and inversion
of configuration are known.40–42 With respect to nucleophiles
and electrophiles similar to the current work, substitution at a
silicon-stereogenic chlorosilane with an achiral lithium silanide
To examine these questions, we devised a three step synthesis
of 13, a precursor to 8 via dearylation-hydrogenation (Scheme
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1).32,44 Uhlig reported 13 in 1993, but synthetic procedures were
accomplished with triflic acid. Magnesium reduction of the
hexasilane framework yielded a mixture of cis- and trans-13.
not provided.45 Coupling of known 9 and 10 in tetrahydrofuran
yielded 11 in 73% yield. Selective terminal bis-dearylation was
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Figure 2. NMR spectra of cis- and trans-13. a) 1H NMR spectra 400 MHz, CDCl3) comparing (top to bottom) trans, cis, and the as isolated
mixture of isomers. Only the methyl region is shown for clarity. b) 29Si {1H} DEPT NMR spectra (79 MHz, CDCl3) comparing (top to
bottom) trans, cis, and the as isolated mixture of isomers.
of diastereomerically pure samples of each, allowing
assignment of the crude mixture arising from reductive
cyclization to a 55:45 trans:cis ratio of diastereomers based on
integration of the 1H NMR spectrum (Figure 2a).
Crystal structures of cis- and trans-13 were determined
(Figure 3). A chair-like conformation of the cyclohexasilane
was observed in both isomers. Typical bond lengths and angles
for cyclohexasilanes were observed. In trans-13, both phenyl
groups were found at equatorial positions, while in cis-13, one
phenyl ring is in the equatorial position while the other is in a
pseudo-axial position.
trans-13 is a chiral, C2 symmetric molecule, with three
chemically inequivalent silicon atoms, and 29Si {1H} DEPT
NMR spectroscopy showed three resonances (Figure 2b). Five
1
resonances were observed in the alkyl region of the H NMR
spectrum and were assigned to the five diastereotopic methyl
groups.Aromatic resonances were also observed. cis-13 is meso
achiral and rapidly interconverting chair conformers were
anticipated. The expected three chemically inequivalent silicon
resonances were observed by 29Si DEPT NMR spectroscopy, as
1
well as five resonances in the alkyl region of the H NMR
spectrum that were assigned to the methyl protons. Peak
assignments are indicated in Figure 2 and are based on 1H-29Si
HMBC spectroscopy (Figures S1-2). We were not able to
distinguish axial and equatorial methyl peaks on the basis of
NOESY spectroscopy.
Figure 3. Displacement ellipsoid plots (50% probability level) of
trans- and cis-13. Black = carbon, blue = silicon at 110(2) K.
Hydrogens are omitted for clarity.
Studies towards the synthesis of 8 began with the as isolated
55:45 trans:cis mixture of 13 (Table 1). Treatment with triflic
acid (TfOH) in dichloromethane (DCM) led to skeletal
rearrangement and an inseparable mixture of structural isomers.
The cis and trans isomers of 13 were partially separable by
column chromatography. Recrystallization of the enriched
samples obtained from column chromatography led to isolation
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Skeletal rearrangement was also observed with acetyl chloride-
aluminum chloride (AcCl/AlCl3). Krempner et al. have
observed skeletal rearrangement of a silane dendrimer in
TfOH/DCM, but not in TfOH/pentane.46 Upon exploring
solvent effects, we observed exclusively cyclohexasilane 14 in
TfOH/pentane (entry 2), but with a change in the diastereomeric
ratio to 24:76 trans:cis-14. Subjecting pure trans-13 to
TfOH/pentane likewise resulted in a 24:76 trans:cis mixture. In
all cases, addition of triethylamine hydrochloride (Et3N•HCl) to
14 provided chlorosilane 8 with no further change in the
isomeric ratio.
MgBr2•[(18-cr-6)K]+ 7, presumably due to the magnesium
silanide’s low solubility in toluene (entry 3). Attempts to
investigate other solvents were thwarted by 14’s poor
compatibility with other solvents: upon addition of 14 to THF,
an insoluble gel formed, presumably due to ring-opening
polymerization of THF.
2
1
2
3
4
5
6
7
8
While coupling 8 with either [(18-cr-6)K]+ 7 or CuCl2•[(18-
2
cr-6)K]+ 7 in THF did not yield the desired product (entries 4-
2
5), MgBr2•[(18-cr-6)K]+ 7 in THF provided Si10Ph4 in 68%
2
yield (entry 6). We also observed successful coupling of [(18-
9
cr-6)K]+ 7 and 8 in toluene, providing the siladecalin in 50%
2
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Table 1. Dearylation of 13: stereochemical consequences.
yield (entry 7). Polymeric material was the major byproduct in
all cases.
With the exception of the heated reaction (entry 2), all
reactions yielded predominantly the cis diastereomer. While
many details of this reaction remain to be elucidated, we suggest
that a combination of the cis enrichment in 8 and 14 and an
intramolecular tethering effect after formation of the first Si-Si
bond contribute to the diastereoselectivity.
Entry
Starting
material
Conditions
Outcome
The ability to isolate samples enriched in the cis-siladecalin
framework encouraged efforts to isolate a high purity sample of
trans-Si10Ph4. We initially attempted separation techniques
similar to those that had succeeded with 13; however, no
separation was observed by silica gel chromatography. Chiral
HPLC separation was also unsuccessful. Recrystallization of
67:33 trans:cis Si10Ph4 in cold hexanes provided 88:12
trans:cis Si10Ph4, but further recrystallizations did not improve
the trans:cis ratio.
1
2
3
55:45
TfOH, CH2Cl2
TfOH, pentane
TfOH, pentane
Skeletal
rearrangement
trans:cis 13
55:45
24:76 trans:cis
14
trans:cis 13
trans-13
24:76 trans:cis
14
Based on these observations, we opted to advance 13 to the
siladecalin scaffold without separating isomers. The as
synthesized 55:45 trans:cis 13 was converted to 24:76 trans:cis
dichlorosilane 8 in 90% yield using TfOH/pentane and
Et3N•HCl (Scheme 2). Under optimized conditions (vide infra),
Table 2. Investigation of Salt Metathesis Synthesis of
Si10Ph4.
24:76 trans:cis 8 coupled to MgBr2•[(18-cr-6)K]+ 7 in THF to
2
yield Si10Ph4 in 68% yield. Si10Ph4 was isolated as a 10:90
mixture of diastereomers, favoring the cis isomer. The
enrichment in the diasteromeric ratio during salt metathesis
merited further investigation.
Entry SMa Countercation Solvent
Yield drb
(trans:cis)
Scheme 2. Synthesis of 10:90 trans:cis Si10Ph4. i) TfOH (2.2
equiv.), pentane, rt followed by Et3N•HCl, Et2O, 90%; ii)
1
14
14
14
8
K
Toluene 10%
Toluene 5%
Toluene 0 %
19:81
MgBr2•[(18-cr-6)K]+ 7, Toluene, rt, 2 h, 68%. rt = room
2
2c
3
K
67:33
temperature.
Mg
K
-
4
THF
THF
THF
0 %
0 %
68%
-
5
8
Cu
Mg
K
-
6
8
10:90
23:77
7
8
Toluene 50%
a SM = starting material. b dr = diastereomeric ratio. c Reaction
performed at 60 °C.
Leaving group, silanide countercation, and solvent were
investigated to determine influence of reaction conditions on
diastereoselectivity (Table 2).38,47,48 Coupling of [(18-cr-
Thermodynamic equilibration ultimately allowed the
isolation of 97:3 trans:cis Si10Ph4. Silanes are known to readily
form pentavalent structures in the presence of anions that serve
as a pathway for inversion of a tetrahedral center (Figure
4a).41,50 Density functional theory (DFT) calculations suggested
that trans-Si10Ph4 is the lower energy isomer by 4.63 kcal mol-
6)K]+ 7 and 14 in toluene provided Si10Ph4 in 10% yield with a
2
19:81 trans:cis ratio (entry 1). The assignment of diastereomers
was based on X-ray crystallography (vide infra). Heating the
reaction to 60 °C did not increase the yield, however, more
trans-siladecalin was observed (entry 2). Magnesium bromide
1
(MgBr2) was added to [(18-cr-6)K]+ 7 to modify the
(Figure 4b). Further evidence that thermodynamic
2
equilibration might be possible included the observation of a
change in isomer ratio upon heating during exploratory studies
towards Si10Ph4 (Table 1, entry 1 vs 2). Hypercoordination is
not expected to induce significant ring strain into the siladecalin
countercation. The composition of this reagent has not been
determined, but Marschner has shown the formation of Si2Mg
species from potassium silanides and MgBr2 (2 SiK + MgBr2 
Si2Mg + 2KBr).49 No reaction was observed between 14 and
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if the exocyclic substituents (methyl and anion) reside in the
Entry
Additive
Time
Equiv.
dr
1
2
3
4
5
6
7
8
axial positions. However, a potential complication of anion-
induced inversion was the known ability of organometallic
reagents such as methyllithium (MeLi) and potassium tert-
butoxide (KOt-Bu) to cleave Si-Si bonds. We therefore targeted
the use of silanide reagents structurally related to Si10Ph4 to
equilibrate cis-Si10Ph4 to trans-Si10Ph4 and avoided
nucleophiles like fluoride that might react irreversibly (Figure
4c).
(trans:cis)
1
None
1 h
-
19:81
19:81
19:81
19:81
97:3
2
KCl
1 h
2.0
3
18-crown-6
1 h
2.0
4
KCl, 18-crown-6
[(18-cr-6)K]+ 7
[(18-cr-6)K]+ 7
[(18-cr-6)K]+ 7
[(18-cr-6)K]+ 7
[(18-cr-6)K]+ 7
[(18-cr-6)K]+ 7
[(18-cr-6)K]+15
[(18-cr-6)K]+15
1 h
2.0
5
3 h
0.25
0.10
0.10
0.10
0.10
0.10
0.50
0.25
2
6
1 h
45:55
53:47
69:31
85:15
92:8
2
9
7
3 h
2
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8
24 h
48 h
72 h
4.33 h
72 h
2
9
2
10
2
11
89:11
92:8
12
Through
a combination of selective synthesis and
postsynthetic equilibration, we discovered routes to the
functionalized siladecalin Si10Ph4 framework enriched in either
the cis or the trans isomer. Each of these isomers could be
further derivatized without significant erosion of the ring fusion
relative stereochemistry (Scheme 3). Dearylation of 10:90
trans:cis Si10Ph4 with TfOH and reduction with LAH provided
15:85 trans:cis Si10H4 in 65% yield. Similar reaction conditions
with 97:3 trans:cis Si10Ph4 provided 95:5 trans:cis Si10H4 in
68% yield.
Scheme 3. Synthesis of cis- and trans-Si10Ph4.
Figure 4. Epimerization Studies. a) Hypothesis: anion-induced
inversion via pentavalent intermediate or transition state. b)
Calculated energy difference between cis-Si10Ph4 and trans-
Si10Ph4 (B3LYP/6-31G(d)). c) Dianion 7 and monoanion 15 used
for epimerization of Si10Ph4
Heating a 19:81 trans:cis sample of Si10Ph4 in toluene alone
or with neutral species such as KCl or 18-cr-6 did not change
the dr (Table 3, Entries 1-4). However, heating a solution of
[(18-cr-6)K]+ 7 and Si10Ph4 to 60 °C for three hours had a
2
dramatic effect on dr (Table 2, entry 5). With 0.25 equivalents
of [(18-cr-6)K]+ 7, a 97:3 trans:cis ratio was reached in ca. 3
2
hours. A lower anion loading (0.1 equivalents of [(18-cr-
2. Siladecalin X-Ray Crystallography. Single crystal X-ray
crystallography facilitated assignment of the cis and trans
isomers of Si10Ph4 and Si10H4. Crystallization of 67:33 trans:cis
Si10Ph4 in cold hexanes provided crystals enriched to 88:12
trans:cis Si10Ph4. Both diastereomers were found in the crystal
structure. The occupancy factor matched the isomer ratio
determined by 1H NMR spectroscopy: the occupancy factor of
the trans-Si10Ph4 isomer (major component) refines to
0.8867(13).
6)K]+ 7) required 72 hours to obtain a 92:8 mixture of trans-
2
Si10Ph4. In both cases, a small amount of an unidentified
byproduct was seen by 1H NMR spectroscopy, which does not
correspond to [(18-cr-6)K]+ 7. Isolated yield with 0.1 loading
2
of [(18-cr-6)K]+ 7 is slightly lower than 0.25 equivalents. [(18-
2
cr-6)K]+15 also epimerized Si10Ph4; however, longer reaction
times (4.33 h) and lower isolated yields (56%) were observed
compared to [(18-cr-6)K]+ 7.
2
Table 3. Anion-Induced Epimerization.
While DFT calculations predicted a chair-chair conformation
for trans-Si10Ph4 in the gas phase (Figure 4b), in the solid state
the SiPh2 ring adopted a chair conformation, while the
permethylated ring adopted a twist-boat conformation (Figure
5a,c). DFT calculations suggested that both the chair-chair and
chair-twist conformers are energetic minima, in which the
chair-chair conformer is lower in energy by 1.19 kcal mol-1
(Figure S3). cis-Si10Ph4 also adopts a contorted geometry in the
solid state (Figure 5b,d). The SiPh2 ring adopted a chair
conformation, but the permethylated ring adopted a half-chair
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conformation. The distortions are presumably to alleviate steric et al. provided evidence that twist-boat conformers contribute
1
2
3
4
5
6
7
8
interactions with the phenyl substituents as Hengge’s crystal
structure of permethyl trans-siladecalin showed a chair-chair
conformation.21
to the room temperature solid state Raman spectrum of
Si6Me12.52
4. Vibrational Spectroscopy. Experimental ATR-FTIR
spectra of neat 15:85 trans:cis Si10H4 and 95:5 trans:cis Si10H4
showed intriguing differences between isomers in the ν(SiH)
region (Figure 7). The 15:85 trans:cis Si10H4 spectrum
displayed two broad stretches at 2069 cm-1 and 2085 cm-1. The
95:5 trans:cis Si10H4 spectrum displayed three sharper
resonances at 2089 cm-1, 2080 cm-1, and 2070 cm-1.
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Figure 5. Displacement ellipsoid plots (50% probability level) of a)
trans- and b) cis-Si10Ph4. Silicon frameworks of c) trans- and d)
cis-Si10Ph4 at 110(2) K. Black = carbon, blue = silicon. Hydrogens
are omitted for clarity.
Crystals of trans-Si10H4 were grown by slowly cooling a hot
acetonitrile solution of a 95:5 trans:cis Si10H4. In the solid state,
trans-Si10H4 adopts a chair-chair conformation (Figure 6). X-
ray diffraction quality crystals of cis-Si10H4 could not be grown.
Figure 7. Experimental ATR-FTIR spectra of 95:5 trans:cis Si10H4
(solid) and 15:85 trans:cis Si10H4 (dashed).
Simulated IR spectra (B3LYP/6-31G(d)) were calculated to
understand how configuration affects the vibrational spectra.
For trans-Si10H4, we considered two conformations: the chair-
chair conformer seen in the crystal structure and a chair-twist
conformer consistent with the crystal structure of Si10Ph4
(Figure 8). Good overall agreement between theory and
experiment was observed, with ν(SiH) resonances predicted to
fall between 2100 and 2050 cm-1. For trans-Si10H4 in either
conformer, the axial and equatorial Si–H bonds resonated at
significantly different frequencies, with the equatorial ν(SiH)
approximately 20 cm–1 lower frequency. These calculations
suggest subtle but real differences in bond strength between
axial and equatorial Si–H bonds in trans-Si10H4. Due to
symmetry, both axial SiH groups have identical stretching
frequencies and the same for equatorial SiH groups. Assuming
that the rings in trans-Si10H4 twist but don’t fully invert, the
experimental FTIR spectrum likely reflects a mixture of chair
and twist isomers in which axial and equatorial SiH bonds have
unique resonances. We found that all four Si–H bonds in cis-
Si10H4 were predicted to have unique resonances due to the
lower symmetry of the cis isomer. The overall greater
conformational heterogeneity expected in the cis-Si10H4 could
explain the broad and featureless experimental IR spectrum.
Figure 6. Displacement ellipsoid plot (50% probability level) of
trans-Si10H4 at 110(2) K. Black = carbon, blue = silicon, pink =
hydrogen. Except for Si-H bonds, hydrogens are omitted for clarity.
Disorder is omitted for clarity.
3. Siladecalin Conformational Analysis. Like carbocyclic
decalin, a key difference between cis- and trans-siladecalin
skeletons is that cis isomers can fully invert both rings
simultaneously, while trans isomers are conformationally
locked. Although the trans siladecalins are not able to invert
both rings simultaneously, we expect that the individual rings
may still contort and sample twist-boat conformers even at
room temperature. This is supported by the X-ray
crystallographic structures above which show both chair and
twist-boat conformations in the solid state.
Prior work has shown that cyclohexasilanes are far more
conformationally flexible than cyclohexanes. NMR studies of
dodecamethylcyclohexasilane (Si6Me12) showed that axial and
equatorial methyl resonances do not resolve until –165 C,
whereas much higher temperatures are typical for cyclohexanes
(ca. –20 C).51 Ab initio calculations predict that the energy
difference between chair and twist conformers of Si6Me12, and
the barrier to twisting is smaller, than for cyclohexane.52 Flock
Experimental ATR-FTIR spectra of diastereomeric Si10Ph4
samples did not show isomer-dependent differences (Figure
S4).
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Table 4. Tabulated UV-vis spectral data.a
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2
3
Compound
λ (nm)
ϵ (L mol-1 cm-1)
235000
trans-Si10Ph4 192
4
5
6
7
8
9
257
45600
cis-Si10Ph4
193
207
240
270
293
194000
trans-Si10H4
74300
21100
9230
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cis-Si10H4
*Onset of Absorption
a Recorded in n-pentane at room temperature.
Absorbance spectra of all four siladecalins were obtained in
n-pentane at room temperature (Table 4). We observed a ca. 20
nm bathochromic shift in cis-Si10Ph4’s onset of absorbance
compared to cis-Si10H4 (315 vs 293 nm) (Figure 9a). Time-
dependent DFT calculations reproduced this trend (TD-
PBE0/6-311G(d)//B3LYP/6-31G(d)) (Figure 9b). The
calculated highest occupied molecular orbital of Si10Ph4
showed minimal electron density on the aromatic rings. Indeed,
the HOMO’s of Si10Ph4 and Si10H4 were markedly similar and
concentrated on the σ-framework (Figure S7). The change in
onset of absorption is attributed to low-lying π* orbitals
centered on the aromatic rings (Figure S7 and Table S1),
lending the Si10Ph4 system more charge transfer character.
Significant differences were also observed between
diastereomeric siladecalins. Both cis- and trans-Si10H4 absorb
in the deep to mid ultraviolet (UV) region, up to ca. 300 nm
(Figure 9c). The absorbance spectrum of trans-Si10H4 had three
distinct transitions at 207 nm, 240 nm, and 270 nm. These
features are similar to the absorbance spectrum of trans-
siladecalin reported by West.20 The absorbance spectrum of
15:85 trans:cis Si10H4 was broad and largely featureless.
Simulated absorbance spectra were calculated (TD-PBE0/6-
311G(d)//B3LYP/6-31G(d)). The onset of absorbance of pure
cis-Si10H4 was predicted to be 15 nm blue-shifted compared to
trans-Si10H4 (Figure 9d, dashed vs. solid black lines line). That
we did not observe this experimentally may reflect the minor
component of trans-Si10H4 present in the synthetic sample. We
found that both a HOMO to LUMO transition and HOMO-1 to
LUMO+1 transition contribute to the broad chair/chair trans-
Si10H4 simulated spectrum (Table 5 and Figure 10). Curious to
understand the origin of the three features in the experimental
spectrum, we also calculated a simulated absorbance spectrum
of trans-Si10H4 in a chair-twist conformation (Figure 9d, gray)
and found an absorption spectrum with a λmax of 223 nm
corresponding to the HOMO-1 to LUMO+1 transition and a
shoulder at 260 nm corresponding to the HOMO to LUMO
transition (Table 5).
Figure 8. Calculated Si10H4 structures and predicted ν(SiH)
frequencies. a) trans-Si10H4 (chair-chair conformer), b) trans-
Si10H4 (chair-twist conformer), and c) cis-Si10H4 (chair-chair
conformer). B3LYP/6-31G(d).
5. UV-Vis Spectroscopy. Oligosilane optical properties are
strongly conformation dependent.53 The HOMO-LUMO gap of
linear silanes decreases as chain length increases, indicative of
-conjugation.54 σ-Conjugation is strongest in an all-anti
conformation.55,56 Deviations from the anti geometry contribute
to short effective conjugation lengths and typical polysilanes
absorb ultraviolet light 350 nm57 and silane dendrimers 290
nm.58
We conclude that the three features in 95:5 trans-Si10H4
spectrum could correspond to the presence of both chair and
twist ring conformers in the synthetic sample. Flock et al. found
evidence
of
dodecamethylcyclohexasilane
twist-boat
conformers at room temperature by solid-state Raman
spectroscopy.52 Likewise, the broadness of the spectrum of
majority cis-Si10H4 could be a consequence of chair-flipping at
room temperature. We finally note that the time-dependent DFT
method used in this study was selected for its prior success in
predicting optical properties of linear silanes61, but a different
method may be optimal for cyclosilanes.
The different conformational profiles of the cis and trans
diastereomers suggested the possibility of stereoisomer-
dependent optical properties. We also expected to observe
differences between aryl- and H-functionalized silanes. The
absorbance spectra of aryl-substituted oligosilanes are generally
bathochromically shifted compared to methylated oligosilanes
due to σ,π-mixing and other effects.54,59,60
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Page 8 of 13
Table 5. Details of Simulated Absorbance Spectra. TD-PBE0/6-311G(d)//B3LYP/6-31G(d).
1
2
3
4
5
6
7
8
Structure
λ (nm) Transition
% Contribution
77%
Oscillator Strength
0.1821
trans-Si10H4 (Chair/Chair)
233.19 HOMO-1  LUMO+1
257.00 HOMO  LUMO
228.32 HOMO-1 LUMO+1
260.21 HOMO  LUMO
221.29 HOMO-4  LUMO
231.24 HOMO  LUMO+1
96%
0.1496
trans-Si10H4 (Chair/Twist)
cis-Si10H4
75%
0.0720
96%
0.0813
66%
0.0432
9
67%
0.0368
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Figure 9. Experimental and simulated absorbance spectra. Comparison of Ph- and H-functionalized siladecalins. (a) Absorbance spectra of
15:85 trans:cis Si10H4 (solid) and 10:90 trans:cis Si10Ph4 (dashed) in n-pentane. [compound] = 5 x 10-6 M. (b) Calculated absorbance spectra
of cis-Si10Ph4 (dashed), cis-Si10H4 (solid). TD-PBE0/6-311G(d)//B3LYP/6-31G(d). Comparison of cis- and trans-Si10H4. (c) Absorbance
spectra of 95:5 trans:cis Si10H4 (solid) and 15:85 trans:cis Si10H4 (dashed) in n-pentane. n-Pentane spectral cutoff is 190 nm. [compound] =
5 x 10-6 M. (d) Calculated absorbance spectra of cis-Si10H4 (dashed), chair/chair trans-Si10H4 (solid black), and chair/twist-boat trans-Si10H4
(solid gray). TD-PBE0/6-311G(d)//B3LYP/6-31G(d).
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are visualized with an isodensity value of 0.04.
6. NMR Spectroscopy. Polycyclic systems can exhibit
unusual NMR spectra. cis-Decalin undergoes rapid ring-
flipping, resulting in coalescence of the signals corresponding
to axial and equatorial substituents. However, trans-decalin
to axial and equatorial SiH’s. NOESY spectroscopy was
inconclusive in assigning each peak to the axial or the equatorial
position.
The splitting patterns provided additional insight facilitating
assignment. Long-range (>3J) 1H-1H coupling in saturated
systems is typically weak, with the exception of rigid cyclic
systems that achieve particularly favorable geometric
alignments.62–64 W-coupling refers to a coplanar, anti-alignment
of H-C-C-C-H bonds that results in detectable long-range
coupling and has been identified in 1,3-functionalized
cyclohexanes and in bicyclo[1.1.1]pentane scaffolds (Figure
12).65,66 Of particular resonance with the present study, Spencer
et al. reported W-coupling as a diagnostic tool in the
identification of cis- and trans-decalins bearing angular methyl
groups: the freely rotating methyl group in trans-10-
methyldecalin has multiple W-coupled pathways with axial
protons, while cis-10-methyldecalin does not.67 This results in
1
does not ring flip and its H NMR spectrum shows closely
overlapping peaks assigned to inequivalent axial and equatorial
protons. 30,31
All cis- and trans-siladecalins isolated herein have sharp
1
1
signals in their H NMR spectra. Cropped H NMR spectra of
cis- and trans-Si10H4 are shown in Figure 11, focusing on the
SiH region of the spectrum. Some remarkable differences are
observed. The cis-Si10H4 has a single sharp peak (Figure 11,
pink). We attribute this to rapid chair-flipping and coalescence
of axial and equatorial SiH groups.
In contrast, the trans-Si10H4 has two distinct resonances: a
doublet (δ 3.42, J = 2.1 Hz) and a multiplet (δ 3.35) (Figure 11,
blue). Both peaks integrate to 2H and are thought to correspond
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significant line broadening of the angular methyl resonance in Conclusions
Page 10 of 13
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the trans isomer compared to the cis.
Herein we describe stereocontrolled syntheses of several
members of a family of cis- and trans-siladecalins. We access
functionalized materials that may serve as useful building
blocks to higher order materials.36,38,68,69 The identity of the
countercation in the key salt metathesis step effects
diastereoselectivity, favoring the cis isomer. Equilibration to
the thermodynamically lower energy trans isomer with an
anionic additive was described. Structural assignments are
supported by X-ray crystallography.
9
Divergent conformational properties are expected for cis and
trans siladecalin scaffolds, which are supported by NMR
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spectroscopy.
Relative
stereochemistry
influences
conformational properties, resulting in a strong effect on optical
properties. The highly flexible cis-siladecalins have broad,
featureless absorption spectra. The less dynamic trans-
siladecalin frameworks have multiple sharp features, which are
proposed to reflect contributions from chair/chair, chair/twist,
and twist/twist conformers.
Stereocontrolled synthesis is a powerful tool for the
preparation of well-defined materials with novel properties. We
expect the synthetic approach outlined herein to facilitate the
synthesis of more examples of cyclosilanes bearing multiple
chiral silicon centers.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
1
Figure 11. Cropped H NMR spectra (400 MHz, C6D6) of 95:5
trans:cis Si10H4 (top) and 15:85 trans:cis Si10H4 (bottom). Only Si-
H region shown for clarity.
Synthetic procedures, tabulated characterization data, copies of 1H,
13C, and 29Si NMR spectra, supplemental figures, details of
experimental procedures, details of computational chemistry (PDF)
X-ray crystallographic data for 13, Si10Ph4, and Si10H4 (CIF)
Original 1H, 13C, and 29Si NMR spectra of new compounds (FID)
AUTHOR INFORMATION
Corresponding Author
* klausen@jhu.edu
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy (DOE),
Office of Science, Basic Energy Sciences under Award # DE-
SC0013906 (building block synthesis, polymerization, and
characterization). DFT calculations were conducted using
scientific computing services at the Maryland Advanced Research
Computing Center (MARCC). We thank Dr. Sravan Surampudi for
helpful discussion.
Figure 12. a) Examples of W-coupling in carbocyclic systems. b)
Conformational analysis of W-coupling in trans-Si10H4. For
clarity, groups not implicated in W-coupling are omitted. 4J = four-
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