Chiral Control in Pentacoordinate Systems: The Case of Organosilicates

Article abstract of DOI:10.1021/acs.inorgchem.8b01861

Chirality at the central element of pentacoordinate systems can be controlled with two identical bidentate ligands. In such cases the topological Levi-Desargues graph for all the Berry pseudorotations (BPR, max. 20) reduces to interconnected inner and out

Full text of DOI:10.1021/acs.inorgchem.8b01861

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Chiral Control in Pentacoordinate Systems: The Case of
Organosilicates
Leon J. P. van der Boon,† Laurens van Gelderen,† Tim R. de Groot,† Martin Lutz,‡ J. Chris Slootweg,†,§
Andreas W. Ehlers,*,†,§,∥ and Koop Lammertsma*,†,∥
†Department of Chemistry and Pharmaceutical Sciences, Faculty of Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1083,
1081 HV Amsterdam, The Netherlands
‡Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht,
The Netherlands
§Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 HX Amsterdam, The Netherlands
∥Department of Chemistry, University of Johannesburg, Auckland Park, Johannesburg 2006, South Africa
S
* Supporting Information
ABSTRACT: Chirality at the central element of pentacoordinate
systems can be controlled with two identical bidentate ligands. In such
cases the topological Levi−Desargues graph for all the Berry
pseudorotations (BPR, max. 20) reduces to interconnected inner
and outer “circles” that represent the dynamic enantiomer pair. High
enough barriers of the BPR crossovers between the two circles is all
what is needed to ascertain chiral integrity. This is illustrated compu-
tationally and experimentally for the organosilicates 7 and 10 that carry
besides a Me (a), Et (b), Ph (c), or F (d) group two bidentate
2-(phenyl)benzo[b]-thiophene or 2-(phenyl)naphthyl ligands, respec-
tively. The enantiomers of tetraorganosilane precursor 9 could be
separated by column chromatography. Their chiral integrity persisted on
forming the silicates. CD spectra are reported for 10c. Fluoro derivative 10d is shown to have its electronegative F substituent in
an equatorial position, is stable toward hydrolysis, and its enantiomers do not racemize at ambient temperatures, while those of
10c racemize slowly.
Berry pseudorotation (BPR) where both axial substituents of a
trigonal bipyramid (TBP) readily exchange with two equatorial
substituents (Figure 1).14,15
Recently though, the stereoselectivity for silicon-centered
nucleophilic substitutions was shown to be controllable by
selectively hampering the BPRs in the pentacoordinate
INTRODUCTION
■
Chirality is a cornerstone of organic chemistry and existential
to all living matter. The very essence of chirality is the non-
superimposability of molecules on their mirror images. Prevalent
in most chiral molecules is the asymmetric tetravalent carbon as
formulated by the first Nobel laureate, J. H. van ‘t Hoff.1,2 The
impact of chirality in organic synthesis, biology, medicine,
polymers, and materials science alike has been monumental.3−6
The dominant role of carbon in chiral systems is complemented
by tetravalent phosphorus, silicon, sulfur, nitrogen, and boron,
but chirality is not limited to molecules deduced from these
elements only.7−9
Chirality of pentacoordinate systems is likewise of inherent
importance and impacts fields like biocatalysis and asymmetric
catalysis.10 However, typically the ligands are responsible for
chirality rather than the transition metal center itself, as in chiral-
at-metal catalysis that has only recently come to the fore.11,12 The
simple reason for the far smaller focus on chiral pentacoordinate
elements in comparison to the tetracoordinated ones is their
dynamic behavior that induces racemization of enantiomers,
which strongly contrast the conformational rigidity of carbon.13
The distinguishing feature is that pentacoordinate systems
are prone to nondissociative racemization by means of the
Figure 1. Berry pseudorotation (BPR, top) and Turnstile rotation
(TR, bottom), which is a double BPR.
Received: July 5, 2018
© XXXX American Chemical Society
A
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Figure 2. Pentaorganosilicate intermediate (left) and isolated
phosphorane (middle; men = menthyl) and pentaorganosilicates
(right).
intermediate 1 shown in Figure 2.16 Akiba and co-workers have
shown in elegant work that high epimerization barriers at phos-
phorus can be obtained using (only) the Martin ligand,17−19
enabling full inhibition of a BPR as was demonstrated by chiral
resolution of 2 and related molecules into their enantiomers
(Figure 2).20 An important factor of the lack of P-epimerization
is the highly electronegative groups in the two axial positions;
we are unaware of other related chiral phosphanes.
These two examples serve to illustrate that chiral integrity is
feasible at the central element of pentacoordinate systems but
also show that it is not evident how to accomplish this. In fact,
the examples might be misleading if apicophilicity is used as a
guiding principle. This then begs the question of how to
achieve chiral integrity for pentacoordinate systems by design.
Here we present the principles how to obtain chiral penta-
organosilicates without the use of chiral ligands.21 As silicates
have modest pseudorotation barriers, smaller than those for
phosphanes, Martin’s ligand does not hamper the BPR in
silicates sufficiently to enable rigidity.22 We provide the con-
ditions needed to obtain any chiral pentacoordinate systems
and present the first chiral pentacoordinate silicon species with
chirality only on the central silicon atom.
The starting point is to recognize how to interfere into the
dynamic conformational behavior of pentacoordinate systems.
Earlier, we reported in detail on the pseudorotations of penta-
organosilicates and pentacoordinate transition metal complexes.23
For clarity and consistency, we summarize and expand on the
findings of this work in order to pinpoint the approach to
achieve chiral integrity.
All feasible pseudorotations for a silicate with five different
substituents can be summarized in a topological Levi−
Desargues graph (Figure 3a), which is the starting point for
our design approach.23−25 Such a system has, in fact, 20 possible
trigonal bipyramidal (TBP) conformations. These are repre-
sented by dots in the Levi−Desargues graph with the numbers
symbolizing the two axial ligands, such as the enantiomeric pair
15 and 51. Each line in the graph represents a BPR. At a
minimum, five such sequential pseudorotations are required for
racemization to occur for any (Δ and Λ) enantiomeric pair.
Inhibiting just one of the BPRs does not suffice, because there
are ample sequences (paths) that connect the enantiomeric
pairs. Controlling all of these daunting task and simplifications
are needed.
The complexity of the Levi−Desargues graph reduces con-
siderably on using two identical bidentate ligands. This is illus-
trated for reported silicate 4, which possesses two 1-phenyl-
pyrrole-2,2′-diyl units and a methyl group (Figure 3b).26 The
number of TBP conformations for this silicate reduces from
20 to 16 or by two for each bidentate ligand. The simple
reason being that one phenylpyrrole group cannot occupy both
Figure 3. (a) Levi−Desargues graph for Berry pseudorotations of
pentacoordinate systems. Unfeasible biaxial conformations for 4 are
red. (b) Two most stable conformations of 4 and unfeasible biaxial
conformation 54 (right). (c) Simplified graph for systems with two
identical bidentate ligands as for 4. Green labeled conformations are
transition structures. The enantiomeric rings are shown in blue and
red, respectively; only TBP conformations are shown. (d) A further
simplified graph for 4 showing only conformational minima is given in
blue, and their relative energies (in parentheses) and conversion
barriers are in black in kcal mol−1.
axial positions simultaneously. Thus, TBP conformer 15 with
the axial phenyl group (labeled 1) and the equatorial pyrrole
group (labeled 5) cannot stereopermutate to TBP conformer
13 and neither can its enantiomer 51 to conformer 31. The
same applies for the other phenylpyrrole group that inhibits
formation of conformers 54 and 45 (see Figure 3b, right).
Eliminating these four (labeled in red) from the Levi−Desargues
graph reduces it to two interconnected double circles (Figure 3c).
The outer one (colored blue) represents one-half of the dynamic
enantiomeric pairs (including Δ-15) and the inner circle (colored
red) contains the enantiomeric counterpart; the two circles are
interconnected by four BPR crossovers. With both bidentate
ligands being identical, only half of the reduced graph needs to
be considered for symmetry reasons, reducing the number of
crossovers to consider (Figure 3d). This simplified representa-
tion reveals three different pathways for racemizing conformers
Δ-15 and Λ-51 with each comprising five subsequent BPRs
(i.e., Δ-15 ⇄ Λ-35 ⇄ Δ-43 ⇄ Λ-51, Δ-15 ⇄ Λ-35 ⇄ Δ-14
⇄ Λ-51, and Δ-15 ⇄ Λ-34 ⇄ Δ-14 ⇄ Λ-51). All three
pathways require barriers high enough to prevent racemization.
The computed values at B3LYP/6-311++G(2d,p) make clear
that this is not the case for 4 (see values in black in Figure 3d).11
The task of designing silicates with chiral integrity then is to
determine which BPRs need to be influenced to inhibit all
three racemization pathways. The clearest choice is to increase
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the two different crossover barriers between the Δ and Λ
enantiomeric rings (blue and red in Figure 3c,d) Extending the
size of the bidentate ligands to induce friction in the pseu-
dorotations is an obvious approach to accomplish this. In fact,
we have attempted this earlier for 3 (Figure 2), which can be
viewed as extended 4, but it did not lead to the desired result.27
However, two diastereotopic CH2 hydrogen atoms could be
observed at −50 °C for the ethyl group of 4b, suggesting only
limited conformational rigidity at that low temperature or below.
All attempts to achieve separation of racemic 4 into its enan-
tiomers were to no avail. Clearly, a still more focused approach
was needed to obtain stable chiral silicates experimentally.
We choose to replace the pyrroles in 3 for phenyl groups
and the naphthyls of 10 for benzo[b]-thiophene groups (7).
The steric effect of Ph-catenated 6-membered phenyl groups
on the silicate is expected to differ from that with the Ph-
catenated 5-membered thiophene groups because of their
different spatial orientations, which should give insight into the
demands for chiral integrity. Exploring these choices of silicates
computationally and experimentally reveal examples of what is
needed to achieve chiral integrity at room temperature. We will
further show that chiral silicates 10 can be obtained by
phenylation/fluorination of their chiral tetracoordinate silane
precursors.
Figure 4. Racemization scheme for methyl,bis-2-(phenyl)-
benzothiophene silicate 7a at B3LYP-GD3/6-311++G(2p,d)//
B3LYP/6-31G(d).
Racemization may then be hampered, and each enantiomer
can still show dynamic behavior if the barriers for connecting
the minima on both the upper and lower parts of Figure 4 are
modest, which seems to be the case. Namely, the one for
converting Δ-15 into Λ-35 amounts to “only” 14.7 kcal mol−1
and involves a Turnstile rotation (TR), which is a single-step
double BPR23,31 with [λ-42]⧧ as the TBP transition structure.
Also, the subsequent conversion of Λ-35 (3.4 kcal mol−1) to
the least stable pseudorotamer Δ-43 (6.9 kcal mol−1) has a
modest barrier of 12.5 kcal mol−1 with TBP [δ-12]⧧ as TS
(Figure 4, upper part). Of course, the entire process is
mirrored for Λ-34 ⇄ Δ-14 ⇄ Λ-51 with [δ-52]⧧ and [λ-32]⧧
as respective TSs. Thus, whereas each enantiomer may be
subject to dynamic behavior in solution at room temperature,
it is still expected that NMR will reveal mainly the most
abundant conformer for 7a, i.e., Δ-15 and/or Λ-51.27
Next we turn to the structures and energies of naphthalene
congener 10 (a: Me; c: Ph; d: F) given in Figure 5 using the
same level of theory; 10b (Et) is omitted due to its similarity
to 10a. Displayed are the Δ-15 ⇄ Λ-35 ⇄ Δ-43 stereo-
permutations of the outer ring of Figure 3c, (analogues to the
upper part of Figure 4 for 7a) and the Δ-43 ⇄ Λ-51
(crossover) path connecting it to the inner ring. Altogether, it
constitutes one of the enantiomeric routes for racemizing Δ-15
and Λ-51, which has, however, a prohibitively high barrier of
27.7 kcal mol−1 for methyl-containing silicate 10a and even
33.7 kcal mol−1 for fluoro silicate 10d. These barriers are even
higher than the corresponding ones discussed for bis-2-
(phenyl)benzo[b]thiophene analogue 7a (24.5 kcal mol−1;
Figure 4) and pyrrole-based 3 (26.6 kcal mol−1).27
RESULTS AND DISCUSSION
■
A computational survey for 7 and 10 is presented first to
evaluate their potential as silicates with chiral integrity, which is
then followed by a description of their synthesis and chiral
properties.
Computational Design. The potential energy surfaces for
both 7 and 10 were examined at B3LYP-GD3/6-311+
+G(2p,d)//B3LYP/6-31G(d). This method has shown to give
good agreements for related, simpler cases;26−29 dispersion
correction was added to account for possible π/π interactions.30
We start by analyzing the data for 7a, which carries a methyl
group and two bidentate 2-(phenyl)benzo[b]thiophene ligands.
Figure 3b gives the numbering scheme of all the Si-substituents
with the TBP conformers labeled by their axial ones. Three
enantiomeric minima were identified of which the Δ-15/Λ-51
pair is energetically favored over the Λ-35/Δ-14 and Δ-43/Λ-34
pairs by 3.4 and 6.9 kcal mol−1, respectively (Figure 4). The
upper part of Figure 4 shows the minima (Δ-15 ⇄ Λ-35 ⇄
Δ-43) that are on the “outer” ring of the topological graph in
Figure 3c,d, and the bottom part shows those of the “inner”
ring (Λ-34 ⇄ Δ-14 ⇄ Λ-51). The identical (enantiomeric)
BPRs paths Δ-15 ⇄ Λ-34 and Δ-43 ⇄ Λ-51 that connect the
outer and inner halves of Figure 3d have the anticipated large
barrier (23.9 kcal/mol) due to steric obstruction, as the ortho-
hydrogens of the phenyl and benzo[b]thiophene moieties of
the two bidentate ligands cannot pass each other unimpeded in
the square planar (SP) transition state (TS); the barrier is
similar to that of 26.6 kcal mol−1 computed for 3.27 The other
BPR path, connecting the enantiomeric minima Λ-35 and
Δ-14, is traversed if the two benzo[b]thiophene moieties can
pass each other in the square planar TS. However, this motion
is obstructed, so significant that the expulsion of one of the
substituents is preferred instead. The barrier for single Si−C
bond cleavage of 30.7 kcal mol−1 concurs with earlier analyses
of pentaorganosilicates.27 With only high energy barriers
connecting the TBPs of the outer and inner halves of Figure 3d,
racemization of chiral 7a may be subdued at room temperature.
Evidently and as expected, the transition states experience
more friction between the α-hydrogens of the two bidentate
naphthyl ligands than for the two benzo[b]thiophenes. The
barrier for the BPR conversion of Δ-43 into Λ-51 is hardly
affected by changing the size of the “fifth” substituent from a
methyl (10a: 27.7 kcal mol−1) to a phenyl group (10c:
28.8 kcal mol−1). The other potential racemization route that
involves Λ-35 ⇄ Δ-14 could not be established as cleavage of
the Si−C bond occurred instead.
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Figure 5. Racemization scheme of methyl,bis-2-(phenyl)naphthylsilicate 10a (orange) and corresponding phenyl 10c (black) and fluorine 10d
(blue) analogues at B3LYP-GD3/6-311++G(2p,d)//B3LYP/6-31G(d).
Whereas racemization may be inhibited, the enantiomers
of naphthylsilicate 10 can display dynamic behavior but to a
lesser extent than discussed for the benzo[b]thiophene homo-
logue 7a for two reasons. First, the Turnstile rotation barrier
([λ−42]⧧) of 18.1 (18.6) kcal mol−1 for converting Δ-15 into
Λ-35 for 10a (10c) is much higher than the 14.7 kcal mol−1
value for 7a. Second, the energy differences of the Δ-15/Λ-51
pair for 10a (10c) with the other Λ-35/Δ-14 and Δ-43/Λ-34
minima of 5.9 (6.9) and 12.3 (12.3) kcal mol−1, respectively,
are much larger than those for 7a (Figure 4). Consequently, it
is expected that only Δ-15/Λ-51 can be observed in solution.
Electronic effects can also play an important role as is evi-
dent on replacing the methyl group of silicate 10a for a fluorine
substituent (10d). As noted, the barrier for the Δ-43 ⇄ Λ-51
conversion increases significantly, which is due to the desta-
bilizing effect of the apical fluorine in the square planar TS.
In contrast, and as expected, the barriers for the Δ-15 ⇄ Λ-35
⇄ Δ-43 Turnstile rotations are much reduced with fluorine in
the apical position of the TBP transition structures [λ−42]⧧
and [δ-12]⧧. In contrast to 10a where the bis-equatorial naph-
thalene ring is π-donating into the Si-Me antibonding orbital,
thereby raising its energy,32 the barriers are much reduced with
the stabilizing apical fluorine (10d). The effect is most
pronounced for conformer δ-12, which in fact becomes a local
energy minimum. Its energy difference of 0.8 kcal mol−1 with
Λ-35 was too small to determine a barrier, nor could the
barrier be found for the formation of conformer Δ-43, which is
likely also very small. In fact, Δ-43 is a transition state for 10d,
but a suitable minimum between this TS and the single Berry
TS (δ-BPR) could not be established. Be that as it may, the
dynamics of the enantiomers of fluorosilicate 10d seems to be
more pronounced than for alkyl and phenyl derivatives 10a
and 10c. Quite interestingly, the global minimum Δ-15 has its
electronegative fluorine substituent in an equatorial instead of
an axial position.27,33
Scheme 1. Synthesis of Sterically Encumbered Silicates
in this case steric factors may favor the triflate despite its low
conversion (35% yield) and the formation of palladium black.
Dilithiation of dibromide 8 followed by reaction with SiCl4
afforded bis(2-phenylnaphthalene-2,1′-diyl)silane 9 in 70%
yield. Further alkylation with MeLi or EtLi, arylation with
PhLi, or fluorination with Me4NF or nBu4NF yielded corre-
sponding silicates 10a−e in almost quantitative yields. The
synthesis of 7 resembles that of 10 and starts with 3-bromo-2-
(2-bromophenyl)benzo[b]thiophene (5, Scheme 1), which
was obtained in 79% yield by a Suzuki coupling of 2,3-
dibromobenzo[b]thiophene and ortho-bromophenylboronic
acid. Treatment of dilithiated 5 with SiCl4 gave silane 635
from which silicates 7a−c were formed by reaction with MeLi,
EtLi, or PhLi. Asymmetric synthesis of the spirosilane pre-
cursors have been described elsewhere.36
The formation of the silicates from the silanes can be
monitored conveniently by 29Si NMR spectroscopy due to the
ca. 90 ppm upfield shift of the Si-resonance. Illustrative are the
87.6 ppm for 7a (δ −111.7; cf., silane 6 δ −24.1) and that of
86.4 ppm for 10a (−95.0; cf., silane 9 −8.6). The presence of
the fluorine atom in 10d is evident from the doublet of 287.8 Hz
in both its 29Si (δ −81.3) and 19F NMR (δ −110.0) spectra;
this 1J(Si,F) coupling is identical, 287.0 Hz, for 10e since both
anions are equivalent. These large coupling constants can be
attributed to the fluorine’s equatorial position in the silicate,27
which would suggest that in solution mainly (or merely) the
Δ-15/Λ-51 conformational pair is observed. In this context, it
Synthesis and Characterization. We now proceed with
the synthesis of silicate 10 first and then 7, following an
established procedure.29 The synthesis of 10 starts with 2-(2-
bromophenyl)-1-bromonaphthalene 8 (Scheme 1) obtained by
a Suzuki coupling between 1-bromo-2-naphthyl triflate and
ortho-bromophenylboronic acid. Whereas this coupling reac-
tion usually performs better with bromides,34 we suspect that
D
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is relevant to note that for each silicate 10a−e and 7a−c, only
a single 29Si NMR resonance was observed, which we assign to
the preferred 15/51 conformers, in concurrence with the
computational results and the sizable energy difference with
their stereoisomers.
1
The H NMR spectra of ethyl substituted 7b and 10b
provide support for their conformational rigidity in solution,
namely, both show diastereotopic methylene protons. The assign-
ment of the axial and equatorial CH2 protons can be derived from
the coupling with carbons 1 and 2 (see Figure 6). The ethyl
Figure 7. Molecular structures of 7c (left) and 10e (right) in their
crystals (ellipsoids are set at 50% probability; hydrogen atoms, lithium
counterion and THF solvent molecules (7c), DCM molecule, and
NnBu4 counterion (10e) are omitted for clarity). Selected bond
lengths [Å] and angles and torsion angles [°] for 7c: Si1−C82
2.0363(13), Si1−C11 1.9395(13), Si1−C13 1.9247(14), C13−Si1−
C11 120.34(6), C13−Si1−C81 92.51(5), C11−Si1−C12 121.50(5),
C11−Si1−C82 94.52(5), C23−C13−Si1−C11 44.26(14). Selected
bond lengths [Å] and angles and torsion angles [°] for 10e: Si1−
C161 1.990(3), Si1−C171 1.921(3), Si1−F1 1.684(2), F1−Si1−
C171 121.39(13), F1−Si1−C161 88.15(12), C171−Si1−C11
115.80(15), C171−Si1−C161 97.97(14).
Figure 6. Silicate 10b with diastereotopic CH2 protons.
group appears to be an excellent diagnostic tool for deter-
mining conformational rigidity. Dynamic silicates like 4-Et give
a coalesced CH2 signal at ambient temperatures,11, but like 3b
(Figure 2), the methylene resonances for more rigid 7b and
10b do not even coalesce at 50 °C, which indicates a barrier of
at least 21 kcal mol−1. While coalescence may then not be
observed on the NMR time scale, exchange experiments (2D-
EXSY) do show the two diastereotopic protons to be corre-
lated at ambient temperatures. This can be attributed to the
modest barriers for the exchange of conformers 15/51, 35/14,
and 43/34 with concurrent rotations of the axial and equatorial
CH2 protons. These results give comfort to the notion that con-
formational rigidity can be obtained by inhibiting a single BPR. In
earlier reported 3b, a Turnstile rotation was inhibited,27 which
seemingly is not needed to prevent racemization (vide infra).
Single crystal X-ray structure determinations were performed
for phenyl containing silicate 7c [racemate] and fluorine sub-
stituted 10e [enantiopure]. Their molecular structures, dis-
played in Figure 7, reveal similar TBP conformations with the
two benzo[b]thiophene groups of 7c and the two naphthyl
groups of 10e in axial positions. As a consequence, the con-
necting phenyl substituents occupy two of the three equatorial
positions in both structures, but although these groups would
ideally have a parallel alignment with the axial bonds for maxi-
mum π stabilization,32 the twist angles of 44.3(14)° for 7c and
48.8(2)° for 10c reflect steric congestion between the phenyl’s
ortho-hydrogen and the α-hydrogens of the benzo[b]thiophene
and naphthyl groups, respectively. Both molecular structures
closely resemble the DFT computed ones for the Δ-15/Λ-51
conformers of 7c and 10e and thereby give credence to these
being the global energy minima. As noted, this is quite a
remarkable observation for 10e as it has the fluorine substituent
in an equatorial position, whereas the dogma is that such a
strong electron-withdrawing substituent should occupy an axial
position. The preference for the equatorial position of the
fluorine atom originates from axial−equatorial aromatic ring
systems, which would otherwise be in the bis-equatorial position
and cause unfavorable π-interaction with the axial bonds.32
Moreover, silicates bearing the SiC4F motif have so far only
been observed in solution or as transient species.27,37−39
The surprisingly stability of silicate 10e with its equatorial
fluorine substituent and nBu4N+ as counterion is further high-
lighted by its resilience to hydrolysis, showing only slow decom-
position in water at a rate of about 10%/h. This behavior is
stunningly different from all other organosilicates, which are
typically extremely sensitive to moisture.28 Only few hydrolyti-
cally stable ones are known.40 Upon hydrolysis of phenyl lithium
silicate 10c, one of its axial bonds cleaves to yield 11 (90−95%
pure) as deduced from the change in mass, the change in 29Si
NMR shift from −99.47 to −10.40 ppm, and the appearance of
a singlet in the aromatic region that is assigned to H1 (Figure 8).
Figure 8. Hydrolysis product 11 from 10c.
It is noteworthy that the counterion markedly influences the
solubility of the silicates. For example, Li+ salts 10a−c are solu-
ble in both THF and DMF, but Me4N+ salt 10d is only soluble
in DMF. nBu4N+ salt 10e dissolves in THF, DCM, and even Et2O.
Chiral Silicates. Our next step was to demonstrate chiral
integrity for one of the organosilicates. On the basis of the
computed racemization barriers for 10 being higher than those
for 7 as well as on the NMR spectroscopic analyses of their
ethyl derivatives, we decided 10 had the best chance to
establish chiral integrity.
We started by separating bis(2-phenylnaphthalene-2,1′-
diyl)silane 9 into its enantiomers with chiral preparative
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HPLC. The molecular structure of Δ-9 [enantiopure],
obtained from a single crystal X-ray structure determination,
is shown in Figure 9. The circular dichroism spectra for both
Figure 11. CD spectra of Δ-10c (solid line) and Λ-10c (dashed line)
in THF (1.9 × 10−4 M) both measured 30 min after their synthesis.
The CD spectra of the enantiomers of 10c show maxima at
266 and 274 nm, akin to 9 but with the same sign, and two
new distinct maxima at 329 and 343 nm, also with the same
sign. The spectra of Λ-10c and Δ-10c show opposite chirality
with the same magnitude for the maxima, thereby suggesting
high optical purity for these enantiomers at ambient temper-
ature. The same was attempted for silicate 10a, which carries a
methyl group instead of a phenyl group. On treating silane Λ-9
with MeLi, the expected product was silicate Δ-10a, but a CD
absorption was absent upon measurement after 30 min, which
was the time needed for sample preparation. The most logical
explanation is that Δ-10a undergoes fast racemization in
solution, albeit that the computational results (no solvation
effects included) show only a 1.1 kcal mol−1 lower barrier for
the racemization barrier (Δ-43 ⇄ Λ-51; Figure 5) than for
10c. Another explanation is that the reaction of MeLi is not as
stereoselective as the one with the bulkier PhLi, causing
racemization during the addition.
Figure 9. Molecular structures of Δ-9 (left) and Λ-10c (right) in their
crystals (ellipsoids are set at 50% probability; hydrogen atoms, lithium
counterion and THF solvent molecule (Λ-10c), are omitted for
clarity). Selected bond lengths [Å] and angles and torsion angles [°]
for Δ-9: Si1−C161 1.8705(15), Si1−C11 1.8607(16), C161−Si1−
C11 91.94(7), C161−Si1−C162 118.37(7), C11−Si1−C12
119.45(7), C11−C61−C71−C161−2.79(19). Selected bond lengths
[Å] and angles and torsion angles [°] for Δ-10c: Si1−C162 2.030(2),
Si1−C12 1.929(2), Si1−C13 1.927(3), C11−Si1−C13 127.77(11),
C162−Si1−C13 90.03(10), C11−Si1−C12 114.67(11), C11−Si1−
C162 94.21(10), C162−Si1−C13−C23−48.80(2).
To assess whether the chiral stability of Δ-10c in solution is
also limited at ambient temperatures, we monitored its CD
spectrum in 10 min intervals, which revealed that the intensity
of the spectrum was reduced by about 50% after a 1 h period
(Figure 12). We attribute this decrease to a slow Berry
Figure 10. CD spectra of Δ-9 (solid line) and Λ-9 (broken line) in
THF (5.6 × 10−5 M).
enantiomers in THF solutions are displayed in Figure 10 and
show the expected opposite Cotton effects with two distinct
maxima at 264 and 279 nm, suggesting high optical purity,
which is in good agreement with the >99% ee observed in
chiral HPLC. As expected, there is no sign of racemization
under ambient conditions. Both silane enantiomers were then
transformed to chiral 10a,c,d silicates using the described
procedure (vide supra). Crystals suitable for a single crystal
structure determination were obtained for Λ10c. Its molecular
structure is given in Figure 9 and shows chiral integrity in the
solid state. Note that the chiral signature changes from Δ for
tetra-coordinate silane 9 to Λ for penta-coordinate silicate 10.
Silicate Λ-10c has a structure similar to 7c and 10e with the
phenyl substituent skewed to the axial bond. The structure
compares well with the DFT computed one for the Λ-15. To
establish whether the chiral integrity is also maintained in
solution we recorded the CD spectra for both enantiomers of
10c in THF solutions (Figure 11).
Figure 12. CD spectra of Δ-10c in THF (6.1 × 0.10−4 M), shown at
10 min intervals. First measurement started 30 min after synthesis of
compound.
pseudorotation for the highest accessible barrier (Δ-43 ⇄
Λ-51). In a separate experiment, Λ-10c was stirred for 1 day at
room temperature, after which no CD signals could be detected.
These observations indicate that 10c undergoes slow racemi-
zation, which is consistent with the computational results.
Interestingly, when phenyl silicate 10c was hydrolyzed after
stirring half an hour at room temperature, isolated hydrolysis
product 11 retained chiral information as shown by the CD
spectra of both enantiomers (Figure 13) with an ee of 33−37%
as determined by HPLC. However, chiral silicates 10c
F
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Inorganic Chemistry
Article
be controlled with two identical bidentate ligands. For such
systems the number of 20 possible trigonal bipyramidal struc-
tures in the topological Levi−Desargues graph reduces to 16 in
two interconnected rings with each ring representing one
enantiomer of the conformationally dynamic enantiomeric
pairs. The two unique crossover paths connecting the two rings
must be inhibited to prevent racemization. The barrier for
these two Berry pseudorotations can be readily increased by
extending the size of the bidentate ligands. We have demon-
strated this computationally and experimentally for organo-
silicates 7 and 10 that carry besides a Me (a), Et (b), Ph (c),
or F (d) group two bidentate 2-(phenyl)benzo[b]thiophene
and 2-(phenyl)naphthyl ligands, respectively. Racemic neutral
silane precursor 9 could be separated into its enantiomers by
column chromatography. Their chiral integrity persisted on
forming the organosilicates. CD spectra were obtained for 10c,
albeit that the solvated enantiomers underwent slow racemi-
zation over time. This loss of chiral information was also
observed in the hydrolysis product of 10c as the ee decreased
progressively when 10c was allowed to stir longer prior to the
addition of water. In sharp contrast, fluoro derivative 10d,
which has its electronegative F group in an equatorial position,
did not show any tendency to hydrolyze nor did its enantiomer
show any indication for racemization.
Figure 13. CD spectra of S-11 and R-11 in THF (1.9 × 10−4 M).
Their absolute conformation could not be determined.
progressively racemize over time, and no ee or CD absorption
was observed when they were reacted toward 11 after stirring
overnight. This observation is in line with the progressive
decline of chiral information for Δ-10c (Figure 12).
We conclude with silicate 10d with its equatorial fluorine
substituent. Its computed barrier of 33.7 kcal mol−1 for racemi-
zation (Δ-15 ⇄ Λ-51) is a significant 4.9 kcal mol−1 higher than
that for 10c. Figure 14 shows the CD spectra with a maximum at
We believe that the principles outlined in this study are
applicable to any pentacoordinate system with nondissociating
ligands and may advance, e.g., chiral-at-metal catalysis for asym-
metric reactions and our general understanding of racemization
in silicon substitution reactions.
EXPERIMENTAL SECTION
■
Computational Methods. All calculations were performed using
Gaussian09D.41 Geometries were optimized at the B3LYP/6-31G(d)
level.42−52 The nature of each stationary point was confirmed by a
frequency calculation. Single-point energies were calculated at the
B3LYP/6-311++G(2d,p) using GD3 as a dispersion correction.30
General. Pyridine was dried using activated molecular sieves
(3 Å); dimethylformamide (DMF) was bought predried from Sigma-
Aldrich and stored on activated molecular sieves (3 Å). Tetrahy-
drofuran (THF) was distilled subsequently from LiAlH4 and sodium/
potassium alloy and diethyl ether from sodium/potassium alloy.
n-Butyllithium and methyllithium were purchased as 1.6 M solutions in
hexanes and in diethyl ether, respectively; ethyllithium was purchased as
a 0.5 M solution in benzene/cyclohexane (9:1). Phenyllithium was
purchased as a 1.9 M solution in di-tert-butyl ether. Tetrachlorosilane
was distilled and refluxed before use to remove HCl. The mass, NMR,
and melting point samples of silicates were prepared and handled in the
purified N2 atmosphere of an MBRAUN Unilab glovebox; other
syntheses were performed using standard Schlenk techniques. NMR
spectra were recorded on a Bruker Avance 400 (1H, 13C, 29Si, 2D
spectra), or Bruker Avanve 500 (1H, 13C). NMR chemical shifts are
internally referenced to the solvent for 1H (DMF: 2.92, CHCl3: 7.26,
THF: 3.58, CH2Cl2: 5.32, DMSO: 2.50 ppm) and 13C (DMF: 34.89,
CHCl3: 77.16, THF: 67.58, CH2Cl2: 53.84, DMSO: 39.52 ppm) and
externally for 29Si (TMS) and 19F (CFCl3). Melting points were
measured on samples in sealed capillaries and are uncorrected.
HR-ESI-MS measurements of silicates were measured on a Varian
IonSpec FT-ICR mass spectrometer. CD measurements were per-
formed on a Chiralscan CD spectrometer, using a 1 mm cuvette
modified for Schlenk techniques. CD of the silicates was measured
half an hour after evaporation of solvent, the silicates were dissolved
in THF or DMF and the solution not used for CD was measured
using NMR to establish purity. Separation of enantiomers was
performed at Syncom Groningen at a preparative scale (500 mg) on a
Chiralpak IA column yielding 195 mg and 145 mg of the single
Figure 14. CD spectra of Λ-10d (solid line) and Δ-10d (broken line)
in DMF (8.4 × 10−4 M).
333 nm for both enantiomers of the fluoride containing silicate;
because of the DMF solvent used, the spectra could only be
recorded down to 285 nm. Monitoring the spectra over time
(>1 day) had no influence on the intensity of the signals. This
observation indicates that silicate 10d does not undergo racemi-
zation and displays chiral integrity at ambient temperatures.
This very fact is in itself rather remarkable. Silicate 11d
with its two all-carbon bidentate ligands and a single fluoride
substituent is the most stable one of those investigated in this
study. Additionally, it carries the electronegative fluoride in an
equatorial position instead of an axial one. Moreover, the
enantiomers of 10d are stable and do not racemize!
Achieving a feat like this bodes well for controlling the chirality
of many more penta-coordinate systems. This systematic study
shows it to be well within the realm of possibilities. It reveals, even
for a fluorosilicate, that chiral integrity can be maintained for
higher coordinate systems in spite of the dynamic behavior of its
enantiomers. We feel that the principles presented herein for
stable silicates with chiral integrity serve as the onset to a much
broader investigation in search of taking advantage of the inherent
chirality of higher coordinate systems and to open up new oppor-
tunities and applications that have been dormant for too long.
CONCLUSION
In summary, we have shown for the element silicon that the
chirality at the central element of pentacoordinate systems can
■
G
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
enantiomers with >99% ee. Chirality was determined using X-ray
crystallography.
to room temperature to stir for another 15 min. Solvents were
evaporated, and a pale yellow solid remained, which was washed using
Et2O (2,5 mL), stripped using THF, and residual solvents evaporated
to obtain 7a as a pale yellow solid (71.7 mg, 0.095 mmol, 84%).
1H NMR (500.23 MHz, THF-d8, 294 K): δ 7.80 (d, 3J(H,H) =
8.0 Hz, 4H; H4, H7), 7.42 (d, 3J(H,H) = 7.0 Hz, 2H; H11), 7.19 (d,
Synthesis. 3-Bromo-2-(2-bromophenyl)benzo[b]thiophene (5).16
3
3J(H,H) = 7.5 Hz, 2H; H14), 7.11 (t, J(H,H) = 7.3 Hz, 2H; H5),
7.04 (t, 3J(H,H) = 7.3 Hz, 4H; H6, H13), 6.69 (t, 3J(H,H) = 7.3 Hz,
2H; H12), 0.44 (s, 3H; H15). 13C{1H} NMR (125.78 MHz, THF-d8,
293 K): δ 170.02 (C2), 152.57 (C10), 148.42 (C3), 147.37 (C9),
146.54 (C1), 145.37 (C8), 137.94 (C11), 128.45 (C13), 127.70
(C4), 124.80 (C12), 123.30 (C5), 122.60 (C7), 122.09 (C6), 120.11
(C14), 7.03 (C15). 1H−29Si-HMBC NMR (400.13, 79.49 MHz,
THF-d8, 296 K): δ −111.7. HR-MS (ESI): calcd for C29H19S2Si
459.0703, found 459.0735. Mp 37.4 °C (decomp.).
Using the method from Lammertsma et al., a solution of 2,3-
dibromobenzo[b]thiophene53 (5.0 mmol, 1.46 g, 1.0 equiv),
2-bromophenyl-boronic acid (5.0 mmol, 1.00 g, 1.0 equiv), Pd(PPh3)4
(0.25 mmol, 289 mg, 0.05 equiv), and Na2CO3 (7.5 mL, 15 mmol, 2 M
in water, 3 equiv) in 50 mL of 1,4-dioxane was heated to 100 °C and
stirred overnight (18 h) under nitrogen atmosphere, resulting in a
black suspension. The solvent was removed under reduced pressure
and the residue dissolved in DCM. The reaction mixture was washed
with water and brine, and the organic layer was dried over MgSO4,
filtered, and concentrated in vacuo, resulting in a yellow oil. The oil
was purified via flash column chromatography (c-hexane), resulting in
an oil. After 3 days, white crystals were formed at 21 °C (1.44 g,
Ethylsilicate (7b).
1
3.9 mmol, 78%). H NMR (500.23 MHz, CD2Cl2, 294 K): δ = 7.88
3
(d, 3J(H,H) = 8.0 Hz, 2H; H4, H7), 7.74 (d, J(H,H) = 8.0 Hz, 1H;
3
H14), 7.53 (t, J(H,H) = 7.5 Hz, 1H; H5), 7.48−7.43 (m, 3H; H6,
3
H11, H12), 7.37 (t, J(H,H) = 7.5 Hz, 1H; H13). 13C{1H} NMR
(125.78 MHz, CD2Cl2, 296 K): δ 138.76 (C8), 138.17 (C3), 137.77
(C9), 134.35 (C1), 133.35 (C14), 132.92 (C11), 131.20 (C13),
127.72 (C12), 126.17 (C6), 125.71 (C5), 124.79 (C10), 123.82
(C4), 122.72 (C7) 108.76 (C2).
EtLi (0.24 mL. 0.118 mmol, 1.1 equiv) was added to a solution of 6
(50 mg, 0.113 mmol, 1.0 equiv) in THF (1 mL) at −78 °C. The
slightly pale suspension was stirred at this temperature for 15 min and
warmed to room temperature to stir for another 15 min. Solvents
were evaporated in vacuo to obtain 7b as a pale yellow foam (73.7 mg,
Benzothiophenesilane (6).
1
0.096 mmol, 85%). H NMR (500.23 MHz, one drop of DMF in
3
THF-d8, 294 K): δ 7.86 (d, J(H,H) = 8.0 Hz, 2H; H4), 7.81 (d,
3
3J(H,H) = 8.0 Hz, 2H; H7), 7.47 (d, J(H,H) = 7.0 Hz, 2H; H11),
7.19 (d, 3J(H,H) = 7.0 Hz, 2H; H14), 7.11 (t, 3J(H,H) = 7.3 Hz, 2H;
3
3
H5), 7.04 (t, J(H,H) = 7.3 Hz, 4H; H6, H13), 6.70 (t, J(H,H) =
7.3 Hz, 2H; H12), 1.16−1.12 (m, 1H; H15eq), 0.90−0.84 (m, 1H;
H15ax), 0.68 (t, 3J(H,H) = 8.0 Hz, 3H; H16). 13C{1H} NMR
(125.78 MHz, one drop of DMF in THF-d8, 293 K): δ 169.37 (C2),
151.68 (C10), 148.64 (C3), 147.58 (C9), 147.08 (C1), 145.29 (C8),
138.18 (C11), 128.48 (C13), 127.88 (C4), 124.89 (C12), 123.23 (C5),
122.56 (C7), 121.99 (C6), 120.10 (C14), 15.90 (C15), 9.75 (C16).
1H−29Si-HMBC NMR (400.13, 79.49 MHz, THF-d8, 296 K): δ −102.1.
HR-MS (ESI): calcd for C30H21S2SiO 489.0803, found 489.0827.
Phenylsilicate (7c).
nBuLi (16.7 mL, 26.7 mmol, 4.1 equiv) was added to a solution of 7
(4.8 g, 13.0 mmol, 2.0 equiv) in Et2O (55 mL) at −78 °C, and the
yellow suspension was stirred at this temperature for 1 h. The reaction
mixture was warmed up to room temperature, and SiCl4 (6.5 mmol,
0.75 mL, 1.0 equiv) was added at 0 °C. The resulting white suspen-
sion was stirred overnight (17 h) at room temperature. The reaction
mixture was washed using water and extracted three times into Et2O.
The combined organic layers were dried over MgSO4, filtered, and
concentrated in vacuo. The resulting colorless solid was further
purified using flash column chromatography (c-hexane), which after
evaporation of all volatiles resulted in a slightly yellow powder (2.03 g,
1
4.56 mmol, 70%). H NMR (500.23 MHz, CD2Cl2, 294 K): δ 7.89
3
3
(d, J(H,H) = 8.0 Hz, 2H; H7), 7.68 (d, J(H,H) = 7.5 Hz, 2H;
3
3
H14), 7.52 (t, J(H,H) = 7.5 Hz, 2H, H13), 7.39 (d, J(H,H) =
7.0 Hz, 2H; H11), 7.25−7.21 (m, 6H; H4, H6, H12), 7.11
(t, 3J(H,H) = 7.5 Hz, 2H; H5). 13C{1H} NMR (125.78 MHz,
CD2Cl2, 296 K): δ 161.26 (C1), 145.76 (C9), 143.46 (C8), 141.55 (C3),
134.23 (C11), 133.57 (C10), 131.93 (C13), 129.81 (C2), 128.58 (C12),
125.36 (C5), 124.77 (C6), 124.66 (C4), 123.38 (C7), 122.43 (C14).
1H−29Si-HMBC NMR (400.13, 79.49 MHz, THF-d8, 296 K): δ −24.1
(Si). HR-MS (EI): calcd for C28H16S2Si 444.0463, found 444.0457. Mp
215.5 °C (decomp.). The asymmetric route is published elsewhere.36
Methylsilicate (7a).
PhLi (0.07 mL. 0.14 mmol, 1.2 equiv) was added to a solution of 6
(50 mg, 0.113 mmol, 1.0 equiv) in THF (1 mL) at −78 °C. The
brown solution was stirred at this temperature for 15 min and warmed
to room temperature to stir for another 15 min. Solvents were
evaporated, and a pale brown solid remained, which was washed using
Et2O (2,5 mL), stripped using THF, and residual solvents were
evaporated to obtain 7c as a pale white solid (73.6 mg, 0.90 mmol,
80%). 1H NMR (500.23 MHz, THF-d8, 294 K): δ 7.88 (d, 3J(H,H) =
3
8.0 Hz, 2H; H7), 7.37 (d, J(H,H) = 8.0 Hz, 4H; H11, H14), 7.26−
7.22 (m, 4H; H4, H13), 7.18−7.16 (m, 2H; o-PhH), 7.04 (t, 3J(H,H)
3
= 7.5 Hz, 2H; H6), 6.94 (t, J(H,H), 2H, H5), 6.82−6.78 (m, 5H,
H12, m-PhH, p-PhH). 13C{1H} NMR (125.78 MHz, THF-d8, 293 K):
δ 168.21 (C2), 151.58 (ipso-PhC), 150.65 (C10), 147.76 (C3),
147.66 (C9), 146.77 (C1), 144.76 (C8), 137.90 (C11), 133.66
(m-PhC), 129.69 (C13), 127.47 (C4), 127.08 (o-PhC), 125.47
(p-PhC), 125.39 (C12), 123.59 (C5), 122.98 (C7), 122.70 (C6),
1
120.60 (C14). H−29Si-HMBC NMR (400.13, 79.49 MHz, THF-d8,
MeLi (0.084 mL. 0.135 mmol, 1.2 equiv) was added to a solution of 6
(50 mg, 0.113 mmol, 1.0 equiv) in THF (1 mL) at −78 °C. The clear
yellow solution was stirred at this temperature for 15 min and warmed
296 K): δ −107.5. HR-MS (ESI): calcd for C34H21S2Si 521.0859,
found 521.0842. Mp 59.4 °C (decomp.).
H
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Article
1-Bromo-2-naphthyl Trifluoromethanesulfonate.17 Using a modi-
fied method from Jarvest et al., triflic anhydride (81.6 mmol, 13.72 mL,
1.2 equiv) was added dropwise to a solution of 1-bromo-2-naphthyl
(68.0 mmol, 15.16 g, 1.0 equiv) in 100 mL of pyridine at
0 °C. After 5 min, the red reaction mixture was warmed to room
temperature and stirred overnight. The reaction mixture was washed
with water, extracted with Et2O (3×), and the combined organic layers
washed with brine, dried over MgSO4, filtered, and concentrated in vacuo.
The resulting brown oil was purified via flash column chromatography
(c-hexane/EtOAc = 19:1), resulting in an yellow oil that crystallized upon
cooling overnight at −5 °C, resulting in off-white crystals (23.2 g, 96%).
H12), 8.09 (d, 3J(H,H) = 8.5 Hz, 2H; H4), 7.81 (d, 3J(H,H) =
8.2 Hz, 2H; 6H), 7.60 (t, J(H,H) = 7.7 Hz, 2H; H13), 7.43 (d,
3
3J(H,H) = 6.9 Hz, 2H; H15), 7.29−7.23 (m, 4H; H7 and H14), 7.16
(d, 3J(H,H) = 8.2 Hz, 2H; H9), 7.06 (t, 3J(H,H) = 7.6 Hz, 2H; H8).
13C{1H} NMR (125.78 MHz, CD2Cl2, 296 K): δ 150.42 (C11),
149.81 (C2), 137.47 (C10), 134.54 (C15), 133.66 (C5), 133.17 (C16),
132.70 (C4), 131.90 (C13), 131.40 (C1), 128.88 (C6), 128.66 (C9),
128.55 (C14), 127.43 (C8), 126.23 (C7), 122.09 (C12), 120.43 (C3).
1H−29Si-HMBC NMR (400.13, 79.49 MHz, CD2Cl2, 296 K): δ −8.64;
IR (neat): 3040 (w, C−HAr), 820 (m, CAr−H), 758 (s, CAr−H). HR-
MS (EI): calcd for C32H20Si 432.1334, found 432.1330. Mp 275.7 °C
(decomp.). The asymmetric route is published elsewhere.36
3
1H NMR (500.23 MHz, (CD3)2SO, 296 K): δ 8.23 (d, J(H,H) =
8.5 Hz, 1H), 8.19 (d, 3J(H,H) = 9.1 Hz, 1H), 8.12 (d, 3J(H,H) = 7.9 Hz,
Bis(2-phenylnaphthalene-2,1′-diyl)-methylsilicate (10a).
3
3
1H), 7.81 (t, J(H,H) = 7.7 Hz, 1H), 7.73 (t, J(H,H) = 7.6 Hz, 1H),
3
7.66 (d, J(H,H) = 9.1 Hz, 1H). Mp 32.2 °C.
2-(2-Bromophenyl)-1-bromonaphthalene (8).
MeLi (1.6 M, 0.139 mmol, 0.09 mL, 1.1 equiv) was added to a solu-
tion of bis(2-phenylnaphthalene-2,1′-diyl)silane (9) (0.116 mmol,
50 mg, 1.0 equiv) in dry THF (1 mL) at −78 °C, and afterward, the
yellow solution was stirred for 30 min at this temperature and 30 min
at room temperature. All volatiles were removed from the clear yellow
solution in vacuo, and the resulting residue was washed with pentane
(3 mL), stripped with THF (1 mL), and dried in vacuo yielding 6a as
A solution of 1-bromo-2-naphthyl trifluoromethanesulfonate (1.5 mmol,
0.53 g, 1.0 equiv), 2-bromophenylboronic acid (1.5 mmol, 0.30 g,
1.0 equiv), Pd(PPh3)4 (0.075 mmol, 86.7 mg, 0.05 equiv), and
Na2CO3 (4.5 mmol, 0.48 g, 3.0 equiv) in 10 mLof 1,4-dioxane and
10 mL of water was heated to 100 °C and stirred overnight (18 h)
under a nitrogen atmosphere. The resulting black suspension was
washed with 2 M HCl and then brine and subsequently extracted with
EtOAc. The combined organic layers were dried over MgSO4, filtered,
and concentrated in vacuo, resulting in a brown oil. The product was
purified via flash column chromatography (c-hexane), and the color-
less oil was recrystallized from pentane, resulting in colorless crystals
1
a yellow solid (88.27 mg, 0.11 mmol, 96%). H NMR (500.23 MHz,
3
THF-d8, 293 K): δ 8.23 (d, J(H,H) = 8.3 Hz, 2H; H9), 8.00 (d,
3
3J(H,H) = 8.3 Hz, 2H; H3), 7.80 (d, J(H,H) = 7.6 Hz, 2H; H12),
7.69 (d, 3J(H,H) = 8.2 Hz, 2H; H6), 7.56 (d, 3J(H,H) = 8.5 Hz, 2H;
H4), 7.31 (d, 3J(H,H) = 6.9 Hz, 2H; H15), 7.15 (t, 3J(H,H) = 6.9 Hz,
2H; H7), 7.08 (t, 3J(H,H) = 7.1 Hz, 4H; H8, H13), 6.61 (t, 3J(H,H)
= 6.9 Hz, 2H; H14) 0.49 (s, 3H; H17). 13C{1H} NMR (125.78 MHz,
THF-d8, 293 K): δ 168.66 (C1), 150.11 (C16), 149.88 (C11), 141.02
(C2), 139.87 (C10), 139.17 (C15), 134.10 (C9), 133.76 (C5),
128.28 (C6), 128.15 (C13), 125.40 (C4), 124.37 (C14), 123.15
(C7), 123.02 (C8), 119.87 (C3), 118.80 (C12), 10.85 (C17).
1H−29Si-HMBC NMR (400.13, 79.49 MHz, THF/THF-d8, 295 K):
δ −95.0 (Si). HR-MS (ESI): calcd for C33H23Si 447.1575, found
447.1595. Mp 33.7 °C (decomp.).
1
(0.180 g, 0.50 mmol, 33%). H NMR (500.23 MHz, (CD3)2SO, 296
3
K): δ 8.27 (d, J(H,H) = 8.3 Hz, 1H; H9), 8.07−8.04 (m, 2H; H4
3
4
and H6), 7.78 (dd, J(H,H) = 8.0 Hz, J(H,H) = 0.8 Hz, 1H; H15),
3
4
7.74 (td, J(H,H) = 7.0 Hz, J(H,H) = 1.3 Hz 1H; H8), 7.67 (td,
4
3
3J(H,H) = 8.0 Hz, J(H,H) = 1.0 Hz 1H; H7), 7.51 (td, J(H,H) =
4
7.6 Hz, J(H,H) = 1.3 Hz, 1H; H13), 7.42−7.37 (m, 3H; H12, H3,
and H14). 13C{1H} NMR (125.8 MHz, (CD3)2SO, 296 K): δ 142.33
(C11), 139.92 (C2), 133.53 (C5), 132.41 (C15), 131.41 (C10),
131.07 (C14), 129.97 (C12), 128.49 (C6), 128.32 (C8), 127.89
(C4), 127.88 (C3), 127.76 (C13), 127.20 (C8), 126.85 (C9), 122.59
(C1), 122.50 (C16). IR (neat): 3053 (w, C−HAr), 1022 (m, C−Br),
955 (m, C−Br), 818 (m, CAr−H), 750 (s, CAr−H). HR-MS (EI):
calcd for C16H10Br2 361.9129, found 361.9129. Mp 69.8−73.0 °C.
Bis(2-phenylnaphthalene-2,1′-diyl)-silane (9).
Lithium Bis(2-phenylnaphthalene-2,1′-diyl)ethylsilicate (10b).
EtLi (0.5 M, 0.122 mmol, 0.24 mL, 1.05 equiv) was added to a
solution of 9 (0.116 mmol, 50 mg, 1.0 equiv) in dry THF (1 mL) at
−78 °C, and the brownish/red solution was stirred for 30 min at this
temperature and 30 min at room temperature. Afterward, all volatiles
were removed in vacuo yielding 6b as a pale yellow foam (93.43 mg,
0.12 mmol, quant.). 1H NMR (500.23 MHz, THF-d8, 293 K): δ 8.28
(d, 3J(H,H) = 8.3 Hz, 2H; H9), 8.00 (d, 3J(H,H) = 8.3 Hz, 2H; H3),
7.80 (d, 3J(H,H) = 7.4 Hz, 2H; H12), 7.70 (d, 3J(H,H) = 8.0 Hz, 2H;
H6), 7.57 (d, 3J(H,H) = 8.3 Hz, 2H; H4), 7.36 (d, 3J(H,H) = 7.1 Hz,
2H; H15), 7.18−7.16 (m, 2H; H7), 7.12−7.07 (m, 4H; H8, H13),
n-BuLi (1.6M, 4.51 mmol, 2.82 mL, 2.05 equiv) was added to a
solution of 8 (2.2 mmol, 0.80 g, 1.0 equiv) in 10 mL of diethyl ether
at −78 °C, and the resulting yellow solution was stirred for 1 h at
−78 °C. The reaction mixture was warmed up to room temperature,
and SiCl4 (1.1 mmol, 0.17 mL, 0.5 equiv) added at 0 °C, and the
resulting white suspension stirred overnight (17 h) at room tem-
perature. The reaction mixture was washed with 2 M HCl, then water
and brine, and subsequently extracted into DCM. The combined
organic layers were dried over MgSO4, filtered, and concentrated
in vacuo. The resulting colorless solid was further purified using flash
column chromatography (c-hexane/EtOAc = 99:1 → 49:1), which
after evaporation of all volatiles resulted in (blue fluorescent) colorless
crystals that were washed with pentane and dried in vacuo (0.33 g,
3
2
6.61 (t, J(H,H) = 7.1 Hz, 2H; H14), 1.34 (dq, J(H,H) = 13.1 Hz,
3J(H,H) = 5.0 Hz, 1H; H17eq), 0.88 (dq, 2J(H,H) = 13.4 Hz, 3J(H,H) =
3
6.4 Hz, 1H; H17ax), 0.33 (t, J(H,H) = 7.4 Hz, 3H; H18). 13C{1H}
NMR (125.78 MHz, THF-d8, 293 K): δ 167.19 (C1), 149.34 (C11),
148.29 (C16), 140.92 (C2), 138.96 (C10), 138.39 (C15), 132.76 (C9),
132.58 (C5), 127.20 (C6), 126.99 (C13), 124.31 (C4), 123.25
(C14), 122.10 (C7), 121.96 (C8), 118.52 (C3), 117.61 (C12), 16.77
(C17), 8.95 (C18). 1H−29Si-HMBC NMR (400.13, 79.49 MHz,
1
0.76 mmol, 69%). H NMR (500.23 MHz, CD2Cl2, 296 K): δ 8.21
3
3
(d, J(H,H) = 8.8 Hz, 2H; H3), 8.13 (d, J(H,H) = 7.6 Hz, 2H;
I
DOI: 10.1021/acs.inorgchem.8b01861
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
THF-d8, 296 K): δ −95.06. HR-MS (ESI): calcd. for C34H25OSi
477.1680, found 477.1702.
Lithium Bis(2-phenylnaphthalene-2,1′-diyl)phenylsilicate (10c).
TBAF (0.12 mL, 0.12 mmol, 1.04 equiv, 1.0 M in THF) was added to
a solution of 9 (50 mg, 0.116 mmol, 1.0 equiv) in THF (1 mL) at
room temperature. The yellow solution was stirred at this temperature
for 30 min and solvents were evaporated afterward until a yellow foam
remained, which was washed with Et2O (2,5 mL) yielding 10e as a
yellow foam (quant.). 1H NMR (400.1 MHz, THF-d8, 294 K):
δ 8.20 (d, 3J(H,H) = 8.0 Hz, 2H; 9H), 7.98 (d, 3J(H,H) =
8.4 Hz, 2H; 3H), 7.72 (d, 3J(H,H) = 7.6 Hz, 4H; 6H, 12H),
7.66 (d, 3J(H,H) = 8.4 Hz, 2H; 4H), 7.20−7.17 (m, 2H; 7H),
3
3
7.09 (t, J(H,H) = 7.4 Hz, 2H; 8H), 7.03 (t, J(H,H) = 7.4 Hz, 2H;
3
3
13H), 6.66 (d, J(H,H) = 7.2 Hz, 2H; 15H), 6.34 (t, J(H,H) =
7.0 Hz, 2H; 14H), 2.73−2.69 (m, 8H; N(CH2Pr)4), 1.25−1.17
(m, 8H; N(CH2CH2Et)4, 1.07−0.98 (m, 8H; N(CH2CH2CH2Me)4,
PhLi (1.9 M, 0.139 mmol, 0.07 mL, 1.2 equiv) was added to a solu-
tion of bis(2-phenylnaphthalene-2,1′-diyl)silane (9) (0.116 mmol, 50 mg,
1.0 equiv) in dry THF (1 mL) at −78 °C, and afterward, the brownish/
red solution was stirred for 30 min at this temperature and 30 min at
room temperature. All volatiles were removed in vacuo, and the resulting
residue was washed with Et2O (2,5 mL) and dried in vacuo, yielding 10c
as a yellowish solid (78.63 mg, 0.98 mmol, 84%). The solid was sus-
pended in 0.7 mL of THF-d8, and two drops of dry DMF were added
to promote solvation allowing NMR measurements. 1H NMR
0.76 (t, J(H,H) = 7.3 Hz, 12H; CH2CH2CH2CH3). 13C{1H} NMR
3
(100.6 MHz, THF-d8, 293 K): δ 160.38 (d, 2J(C,F) = 101.9 Hz, C1),
150.03 (C11), 148.17 (C16), 142.00 (C2), 139.39 (C10), 137.31
(C15), 134.32 (C9), 134.10 (C5), 128.78 (C13), 128.44 (C6),
126.43 (C4), 125.51 (C14), 123.59 (C8), 123.72 (C7), 119.84 (C3),
118.95 (C12), 58.73 (CH2), 24.17 (CH2), 20.09 (CH2), 13.75
(CH3). H−29Si-HMBC NMR (400.13, 79.49 MHz, THF-d8, 296 K):
1
3
δ −88.80 (d, 1J(Si,F) = 283.8 Hz). 19F{1H} NMR (235.3 MHz,
THF-d8): δ −107.04 (1J(F,Si) = 287.0 Hz, 2J(F,C) = 101.9 Hz).
HR-MS (CSI): calcd for C32H20FSi 451.1318, found 451.1297.
11-(2-(Naphthalen-2-yl)phenyl)-11-phenyl-11H-benzo[b]-
naphtho[2,1-d]silole (11).
(500.23 MHz, THF-d8, 294 K): δ 8.05 (d, J(H,H) = 8.5 Hz, 2H;
H3), 7.88 (d, 3J(H,H) = 7.5 Hz, 2H; H12), 7.71 (d, 3J(H,H) = 8.0 Hz,
2H; H9), 7.60−7.56 (m, 4H; H4, H6), 7.14 (d, 3J(H,H) = 7.0 Hz, 2H;
3
H14), 7.11 (t, J(H,H) = 7.5 Hz, 2H; H13), 6.97−6.96 (m, 4H; H7,
o-PhH), 6.68 (t, 3J(H,H) = 7.5 Hz, 2H; H8), 6.58 (t, 3J(H,H) = 7.0 Hz,
2H; H14), 6.54−6.48 (m, 3H; p-PhH, m-PhH). 13C{1H} NMR
(125.78 MHz, THF-d8, 293 K): δ 166.56 (C1), 156.09 (ipso-PhC),
150.85 (C11), 148.50 (C16), 141.80 (C2), 139.70 (C10), 139.38 (C15),
135.61 (o-PhC), 133.95 (C9), 133.62 (C5), 128.41 (C13), 127.75 (C6),
125.96 (C4), 125.90 (m-PhC), 124.57 (C14), 124.09 (p-PhC), 123.02
1
(C7), 122.80 (C8), 119.17 (C3) 118.84 (C12). H−29Si-HMBC NMR
(400.13, 79.49 MHz, THF/THF-d8, 295 K): δ −99.47. HR-MS
(ESI): calcd for C38H25Si 509.1731, found 509.1709.
Tetramethylammonium Fluorosilicate (10d).
PhLi (1.9 M, 0.81 mmol, 0.04 mL, 1.1 equiv) was added to a solution
of bis(2-phenylnaphthalene-2,1′-diyl)silane (9) (0.074 mmol, 32 mg,
1.0 equiv) in dry THF (1 mL) at −78 °C, and afterward, the brownish/
red solution was stirred for 30 min at this temperature and 30 min at
room temperature. All volatiles were removed in vacuo and the resulting
residue was washed with Et2O (2,5 mL) and dried in vacuo. The
residue was dissolved in THF (3 mL) and water (3 drops) was added to
the yellow solution. All volatiles were evaporated to yield the product
1
of >90% purity as a white foam (53 mg). H NMR (400.1 MHz,
Tetramethylammonium fluoride (21.5 mg, 0.23 mmol, 2.0 equiv) was
dried under dynamic vacuum for 3 h at 148 °C. 9 (50 mg, 0.116 mmol,
1.0 equiv) was dissolved in DMF separately and layered on top
of tetramethylammonium fluoride. The yellowish suspension was
stirred for 30 min and filtered afterward. The clear yellow filtrate was
concentrated in vacuo to yield 10d as a white solid (quant.). 1H NMR
(400.1 MHz, DMF-d7, 294 K): δ 8.22 (d, 3J(H,H) = 9.9 Hz, 2H; H9),
CD2Cl2, 294 K): δ 7.93 (d, 3J(H,H) = 7.4 Hz, 1H), 7.78−7.72
(m, 2H), 7.65−7.60 (m, 3H), 7.54 (t, 3J(H,H) = 7.4 Hz, 1H), 7.48−
3
3
7.19 (m, 14H), 7.11 (t, J(H,H) = 7.2 Hz, 1H), 7.02 (d, J(H,H) =
3
8.3 Hz, 1H), 6.98 (s, 1H), 6.87 (d, J(H,H) = 8.1 Hz, 1H), 6.79
(d, J(H,H) = 8.3 Hz, 1H). 13C{1H} NMR (100.6 MHz, CD2Cl2,
3
293 K): 151.47 (q), 148.34 (q), 147.41 (q), 140.08 (q), 138.55 (CH),
137.65 (q), 136.68 (q), 136.03 (2CH), 135.31 (q), 134.29 (q), 133.42
(CH), 133.37 (q), 132.51 (q), 132.18 (q), 132.00 (q), 130.88 (CH),
130.45 (CH), 130.17 (CH), 130.07 (CH), 130.05 (CH), 129.52 (CH),
129.02 (CH), 128.60 (CH), 128.42 (2CH), 128.15 (CH), 127.58
(CH), 127.28 (2CH), 127.04 (CH), 126.88 (CH), 126.44 (CH),
125.83 (CH), 125.71 (CH), 125.64 (CH), 121.38 (CH), 120.00 (CH).
1H−29Si-HMBC NMR (400.13, 79.49 MHz, CD2Cl2, 296 K): δ −10.40.
HR-MS (FD): calcd for C38H26FSi 510.18038, found 510.18183.
3
4
8.03 (dd, J(H,H) = 8.8 Hz, J(H,H) = 2.0 Hz, 1H; H3), 7.80 (d,
3J(H,H) = 8.0 Hz, 4H; H6, H16), 7.73 (d, J(H,H) = 8.4 Hz, 2H;
3
H4), 7.25 (t, 3J(H,H) = 6.8 Hz, 2H; H7), 7.16 (t, 3J(H,H) = 8.0 Hz,
3
4
2H; H8), 7.08 (dt, J(H,H) = 7.4 Hz, J(H,H) = 1.4 Hz, 2H; H15),
3
4
6.60 (dd, J(H,H) = 7.0 Hz, J(H,H) = 0.8 Hz, 2H; H13), 6.54 (dt,
3J(H,H) = 7.1 Hz, 4J(H,H) = 0.8 Hz, 2H; H14), 3.34 (s, 12H, NMe4).
13C{1H} NMR (100.6 MHz, DMF-d7, 293 K): δ 160.28 (C1), 150.51
(C11), 148.87 (C12), 141.94 (C2), 139.66 (C10), 137.15 (C13),
134.15 (C9, C5), 129.62 (C15), 128.92 (C6), 127.14 (C4), 125.81
(C14), 124.69 (C8), 124.64 (C7), 120.24 (C3), 119.72 (C16).
1H−29Si-HMBC NMR (400.13, 79.49 MHz, DMF-d7, 296 K): δ
ASSOCIATED CONTENT
* Supporting Information
■
S
1
−81.31 (d, J(Si,F) = 287.8 Hz). 19F{1H} NMR (235.3 MHz, DMF-
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.8b01861.
d7): δ −109.99 (1J(F,Si) = 289.3 Hz).
Tetrabutylammonium Fluorosilicate (10e).
All 1H, 13C, and 1H−29Si-HMBC NMR spectra.
Geometries of computed structures. X-ray crystallo-
graphic data for 7c, Δ-9, Λ-10c, and 10e (PDF)
Accession Codes
CCDC 1853537−1853540 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
J
DOI: 10.1021/acs.inorgchem.8b01861
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Generating Silicon-Centered Chirality. J. Am. Chem. Soc. 2015, 137
(13), 4304−4307.
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.
(17) Kojima, S.; Nakamoto, M.; Akiba, K. Stereospecific
Pseudorotation of Diastereomeric Anti-Apicophilic Spirophosphor-
anes: A Novel Stereochemical Transformation Involving 10-P-5
Phosphoranes. Eur. J. Org. Chem. 2008, 2008 (10), 1715−1722.
(18) Kojima, S.; Nakamoto, M.; Matsukawa, S.; Akiba, K.
Stereomutation of a Diastereomeric Pair of 10-P-5 Hydroxyphosphor-
anes. Heteroat. Chem. 2011, 22 (3−4), 491−499.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: A.W.Ehlers@uva.nl.
*E-mail: K.Lammertsma@vu.nl.
(19) Kojima, S.; Kajiyama, K.; Nakamoto, M.; Akiba, K. First
Characterization of a 10-P-5 Spirophosphorane with an Apical
Carbon−Equatorial Oxygen Ring. Kinetic Studies on Pseudorotation
of Stereoisomers. J. Am. Chem. Soc. 1996, 118 (50), 12866−12867.
(20) Kojima, S.; Kajiyama, K.; Akiba, K. Characterization of
Enantiomeric Pairs of Optically Active 10-P-5 Phosphoranes with
Asymmetry Only at Phosphorus. Bull. Chem. Soc. Jpn. 1995, 68 (7),
1785−1797.
ORCID
J. Chris Slootweg: 0000-0001-7818-7766
Koop Lammertsma: 0000-0001-9162-5783
Notes
The authors declare no competing financial interest.
̈
(21) Bohme, U.; Fels, S. A New Type of Chiral Pentacoordinated
ACKNOWLEDGMENTS
Silicon Compounds with Azomethine Ligands Made from Acetyla-
cetone and Amino Acids. Inorg. Chim. Acta 2013, 406, 251−255.
(22) Stevenson, W. H.; Wilson, S.; Martin, J. C.; Farnham, W. B.
Pseudorotational Mechanism for the Inversion of 10-Si-5 Siliconates:
Ligand Structure and Reactivity. J. Am. Chem. Soc. 1985, 107 (22),
6340−6352.
■
This work was supported by The Netherlands Organisation
for Scientific Research, Chemical Sciences (NWO−CW).
We acknowledge SARA Computing and Networking Services
for computer time.
(23) Couzijn, E. P. A.; Slootweg, J. C.; Ehlers, A. W.; Lammertsma,
K. Stereomutation of Pentavalent Compounds: Validating the Berry
Pseudorotation, Redressing Ugi’s Turnstile Rotation, and Revealing
the Two- and Three-Arm Turnstiles. J. Am. Chem. Soc. 2010, 132
(51), 18127−18140.
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DOI: 10.1021/acs.inorgchem.8b01861
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