Chiral Memory in Silylium Ions

Article abstract of DOI:10.1002/chem.201501987

The configurational stability of the chiral silicon center in Lewis base-stabilized silyliums has been studied. Complete retention of configuration at silicon is observed at low temperature in silylium ions stabilized by lone-pair interactions. In contras

Full text of DOI:10.1002/chem.201501987

DOI: 10.1002/chem.201501987
Full Paper
&
Chirality |Hot Paper|
Chiral Memory in Silylium Ions
P. Ducos, V. Liautard, F. Robert,* and Y. Landais*[a]
Abstract: The configurational stability of the chiral silicon
center in Lewis base-stabilized silyliums has been studied.
Complete retention of configuration at silicon is observed at
low temperature in silylium ions stabilized by lone-pair inter-
actions. In contrast, loss of chiral memory is observed with
systems involving p-interactions. Epimerization of the silicon
center was observed by simply allowing equilibration of the
diastereomeric silyliums at RT or by adding a catalytic
amount of a chelating solvent. Conditions for the transfer of
axial chirality to Si-centered chirality were shown to rely on
the strength of the Lewis base-silylium ion interaction.
Introduction
Silylium cations have long been considered as elusive species
due to their exceptional reactivity.[1] The poor steric shielding
of the silicon center due to the long bonds to silicon and the
reluctance of silicon to establish p-conjugation explain in part
this high reactivity, which has retarded their isolation until
2002, when Lambert, Reed, and co-workers[2] reported the X-
ray structure determination of a true trivalent silicon cation
(e.g., Mes3Si+ ÀHCB11Me5Br6). The electrophilicity of silylium cat-
ions, however, represents a major asset for their utilization in
catalysis. Recent reports have demonstrated the efficiency of
silylium ions as catalysts in Diels–Alder reactions,[3] in hydrode-
fluorination of alkyl fluorides[4] and as sterically demanding
Lewis acids in frustrated Lewis pair (FLP)-catalyzed hydrogena-
tion reactions.[5] However, the potential of application of these
cations in asymmetric catalysis has been poorly exploited so
far, likely a result of their too-high Lewis acidity.[6] For silylium
ions reacting with any s- or p-nucleophiles, the silicon center
needs to be pacified if one wishes to exploit their unique reac-
tivity in processes where standard Lewis acids are ineffective.
Pioneering studies by Siegel et al.[7] have shown that intramo-
lecular interactions with Lewis-basic substituents (LB=Ar, F, Cl,
Figure 1) temper this electrophilicity, leaving a pyramidalized
silicon center, which however retains most of its positive
charge. Adjustment of the electron-donating ability of the
Lewis base should thus allow control of the acidity of the sili-
con center, as suggested by recent reports[6–9] using various
Lewis-basic (LB) substituents (s-, p-, lone-pair donors or
metals). Interestingly, the labile nature of such weak interac-
Figure 1. Chiral memory at silicon in Lewis-base-stabilized silylium cations.
tions should also favor the regeneration of the catalyst during
the catalytic cycle. Evaluation of the magnitude of these weak
interactions as a function of the nature of the LB groups is
thus important in the context of a future utilization of these
cations in catalysis. It was anticipated, based on the known
pyramidalization of the silicon center in LB-stabilized silyli-
ums,[1,7] that the evaluation of the configurational stability of
a stereogenic silicon center (R1 ¼ R2, Figure 1) would provide
a means to estimate the strength of such LBÀSi+ interaction.
We thus designed a simple model I, incorporating an axial and
a Si-centered chirality, anticipating that a strong interaction in
silylium II should restrict the rotation around the CÀSi bond
and thus lead to retention of chirality at Si, whereas a weaker
interaction might allow an epimerization. Abstraction of the
hydrogen of the silane I, following the Bartlett–Condon–
Schneider hydride transfer reaction[10] would provide the chiral
silylium II, which upon treatment with a hydride source would
return the starting silane. Comparisons between configurations
of the starting and final silanes should thus allow us to draw
conclusions on the extent of chiral memory at silicon. Informa-
[a] P. Ducos, Dr. V. Liautard, Dr. F. Robert, Prof. Dr. Y. Landais
Institute of Molecular Sciences, UMR-CNRS 5255
University of Bordeaux 351, Cours de la libØration
33405 Talence cedex (France)
E-mail: f.robert@ism.u-bordeaux1.fr
y.landais@ism.u-bordeaux1.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501987.
Chem. Eur. J. 2015, 21, 11573 – 11578
11573
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
tion relative to the configuration of the silylium ion II should
1
also be obtained relying on 29Si and 2D H NMR. We disclose
here our preliminary investigations on the nature and strength
of the interaction between the silylium cation and Lewis-basic
substituents such as alkoxy groups (lone-pair interactions) and
arenes (p-interactions) in binaphthyl substrates I and the con-
sequences on the chirality at the silicon center. The conclu-
sions of this work have a broad significance as the strategy
provides a way to pacify highly reactive cationic silicon species
and open a route to novel chiral silylium cations. These studies
also show that cations II retain sufficient Lewis acidity for fur-
ther use in asymmetric catalysis. Finally, epimerization at the
silicon center was also performed under mild conditions, dem-
onstrating that a complete transfer from axial to central chirali-
ty is achievable in these silanes.
Results and Discussion
Chiral silanes 3 and 4 were easily prepared starting from readi-
ly available binaphthyl bromides 1–2 through a two-step se-
quence including a bromine–lithium exchange, followed by
a silylation with Me(tBu)SiClH. The silylation proceeded with
moderate diastereocontrol to afford chiral racemic 3 and 4 as
amorphous materials (for clarity, only one enantiomer is
shown). The relative configuration of the major stereoisomer
Scheme 1. Preparation of chiral binaphthylsilanes 3 and 4.
1
4a was assigned using H NMR (NOE) and established unam-
biguously through X-ray diffraction studies (XRD) of the corre-
sponding crystalline silanol (supporting information). Configu-
1
rations of 3a and 3b were assumed based on H NMR (NOE)
and on the above XRD studies. In parallel, the minor stereoiso-
mer 4b was obtained from 4a, through a two-step sequence
involving a chlorination then a reduction of the corresponding
chlorosilane, proceeding respectively with retention and inver-
sion of configuration at the silicon center.[11]
With silanes 3 and 4 in hand, the desired silylium cation 3a+
and its diastereomer 3b+ were prepared through hydrogen
transfer using Ph3C+[B(C6F5)4]À as a reagent. Starting from a 3:1
mixture of 3a/3b, the corresponding silylium ions were ob-
tained at À788C with the same ratio in a quantitative yield
(1H NMR). Their relative configurations were assigned through
NOE experiments of the crude reaction mixture (Scheme 2).
29Si NMR chemical shifts were those expected for a silylium ion
interacting with an oxygen lone pair (cyclic silyloxonium),[12]
1
implying that oxygen is also stereogenic. Interestingly, H NMR
Scheme 2. Preparation of silylium cations 3a+ and 3b+.
of the mixture led to only two sets of chemical shifts, consis-
tent with the presence of only two diastereoisomers with op-
posite stereochemistry at silicon and the same stereochemistry
at oxygen. Addition of tin hydride to the mixture of 3a+/3b+
at À788C did not return the starting silanes 3a,b but instead
produced a mixture of siloxane 5, the structure of which was
unambiguously assigned through XRD studies, and silanols 6.
Interestingly, yields of 5 and 6 reflect the ratio of the starting
silanes. Siloxane 5 likely results from an attack of tin hydride
onto the highly electrophilic O-alkyl substituent of the major
silyloxonium 3a+,[13] whereas 6 is formed through hydrolysis of
3b+, Bu3SnH not reacting with the silicon center or the OMe
substituent in this case, probably for steric reasons.
When a stronger reducing agent such as LiAlH4 was used at
À788C, the 3a+/3b+ mixture returned the same 3:1 ratio of si-
lanes 3a/3b. Reaction of the 4.5:1 mixture of silanes 4a/4b
with Ph3C+[B(C6F5)4]À also provided the corresponding silyliums
4a+/4b+, the configuration of which was established through
NOE experiments (see the Supporting Information), but addi-
tion of Bu3SnH returned quantitatively 4a/4b with the same
ratio, and no trace of siloxane 5. In parallel, hydrogen transfer
from a 1:8 mixture of silanes 4a/4b (Scheme 1) followed by re-
action with Bu3SnH under the same conditions also led back to
Chem. Eur. J. 2015, 21, 11573 – 11578
www.chemeurj.org
11574
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
4a/4b with the same 1:8 ratio. All these experiments thus
demonstrate that an overall retention of configuration at sili-
con is observed, during the formation of silylium ions from si-
lanes 3 and 4, illustrative of a strong Si+–lone-pair interaction
under these conditions, which prevents any rotation around
the arylÀSi bond.
Table 1. Epimerization at the silicon center in silylium ions 3+/4+.
Entry Silane[a] d.r.[b]
T [8C]
Solvent[c] Product[e] d.r.[b]
1
2
3
4
5
4a/4b
4a/4b
3a/3b
3a/3b
3a/3b
4.5:1 20
1:8 20
3:1 20
1,2-Cl2Ph 4a+[f]
1,2-Cl2Ph 4a+[f]
10:1[g]
>10:1[g]
3:1[h]
>2:98[i]
>98:2[j]
CD2Cl2
CD2Cl2
3a/3b
3b
[d]
[d]
>2:98 À78
Relative energies of silylium species 3a+ and 3b+ (and 4a+
vs. 4b+) were also computed at the DFT level (M06-2X/Def2-
TZVPP), showing that steric interactions between the substitu-
ent on the oxygen (Me or Ph) and the largest group (tBu) on
the silicon center in isomers 3b+ and 4b+ (as shown in NOE
experiments, Scheme 2) led to a weaker O–Si interaction, as in-
dicated by the longer distance between O and Si centers
(Figure 2). Difference in energy between silylium stereoisomers
>2:98 À78/À20 CD2Cl2
3a
[a] The starting silane in a given solvent was treated with 1 equiv of
Ph3C+[B(C6F5)4]À and the reaction mixture stirred at the indicated temper-
ature before being treated with LiAlH4 at À788C. [b] Diastereomeric ratio
of the silane estimated by 1H NMR spectroscopy. [c] 1,2-Cl2Ph: 1,2-dichlo-
robenzene. [d] tBuOMe (0.1 equiv) was added to the mixture. [e] Silane re-
covered after addition of LiAlH4. [f] Addition of LiAlH4 did not return
silane 4a and left 4a+/4b+ unchanged. [g] Diastereomeric ratio of silyli-
um 4a+/4b+ estimated by 1H NMR spectroscopy. [h] Quant. yield. [i] 92%
Yield. [j] 79% Yield.
The same sequence was then applied to a 1:8 mixture of si-
lanes 4a/4b (entry 2, Table 1). As before, a complete epimeri-
zation was observed, in good agreement with computational
studies above. When the same strategy was applied to a 3:1
mixture of 3a/3b, no epimerization was observed at 208C
(entry 3, Table 1), emphasizing the stronger O–Si interaction in
silylium 3a+ compared to 4a+ (Figure 2). Addition of LiAlH4 to
this mixture returned silanes 3a/3b with the same ratio. In
order to facilitate the epimerization, a catalytic amount of
tBuOMe was added, anticipating that an aliphatic ether would
more strongly coordinate to the silylium center and disrupt
the intramolecular O–Si interaction. When diastereomerically
pure 3b in CD2Cl2 was treated with the trityl reagent at À788C
in the presence of 0.1 equiv of tBuOMe, then quenched with
LiAlH4, the silane 3b was recovered in 92% yield (entry 4,
Table 1). Pleasingly, warming the reaction mixture prior to the
addition of the hydride led in quantitative yield to diastereo-
merically pure silane 3a, indicating that a complete epimeriza-
tion had occurred (entry 5, Table 1). These experiments thus
demonstrate that varying the nature of the Lewis-basic sub-
stituent on the aromatic ring allows an efficient tuning of the
intramolecular interaction with the silylium center and that
conditions may be found where this interaction may be dis-
rupted to allow epimerization of the silicon center.
Figure 2. Relative energies of silylium ions 3a+ and 3b+ and 4a+ and 4b+.
was also much higher in the OPh series compared to the OMe
series, as a result of a larger steric interaction between Ph and
tBu substituents (arrow, Figure 2). A longer OÀSi bond is also
noticeable in the OPh series, indicating a weaker interaction,
likely resulting from the electron-withdrawing effect of the
phenyl group. The observed 29Si NMR chemical shifts for 4a+
and 4b+ (57.2 and 55.0 ppm, respectively), slightly higher than
for 3a+ and 3b+, are also in line with these computational re-
sults.
Having shown the influence of the interaction of a lone pair
with a chiral silicon center in silyliums, we extended the con-
cept to the study of p-interactions,[7,8e] using silane 7, which
has
a phenyl substituent as a Lewis basic component
(Scheme 3). Surprisingly, and in contrast with silanes 3 and 4,
silylation of the corresponding aryl bromide led to the stereo-
isomer 7b, and no trace of 7a.[14] Treatment of 7b with Ph3C+
[B(C6F5)4]À led to silylium 7b+, the structure of which could not
Based on the computational studies above, and given the
energy differences between stereoisomers in both series, it
was foreseen that epimerization at the silicon center might be
possible under certain conditions.[9] We first treated a 4.5:1
mixture of silanes 4a/4b with the above trityl reagent at 08C
in 1,2-dichlorobenzene and then allowed the solution contain-
ing the silyliums 4a+/4b+ (in the same ratio (1H NMR)) to
1
be determined directly through H or 29Si NMR[15] due to its in-
stability at temperatures ranging between À50 to 208C
(Scheme 3). Stabilization of 7b+ through addition of 1 equiv of
diethyl ether[16] at À788C, nevertheless allowed the structure
determination of the corresponding silyloxonium ion through
1H and 13C NMR studies. 29Si NMR signals at 53.0 and 51.9 ppm
also showed that the 7b+-ether siloxonium was present with
its stereoisomer 7a+-ether complex, indicating that epimeriza-
1
reach room temperature. After 1 h, H NMR of the crude reac-
tion mixture indicated that epimerization had occurred to pro-
vide the silyliums 4a+/4b+ in a 10:1 ratio (entry 1, Table 1).
Chem. Eur. J. 2015, 21, 11573 – 11578
www.chemeurj.org
11575
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Scheme 3. Structure of silane 7b and preparation of silylium cation 7b+.
Table 2. Epimerization at the silicon center in silylium ions 7+.
Figure 3. Chiral memory at the silicon center in silyliums.
Entry
Silane[a]
d.r.[b]
T [8C]
Ether
Product
d.r.[b]
pared from 7b, relying on the chlorination–reduction se-
quence used for 4b (Scheme 1).
1
2
3
4
5
7a/7b
7a/7b
7a/7b
7a/7b
7a/7b
>2:98
60:40
>2:98
>2:98
>2:98
À78
–
–
7a/7b
7a/7b
7a/7b
7a/7b
7a/7b
>2:98[c]
6:94[d]
71:29[d]
10:90[e]
17:83[f]
À78
A tentative picture summarizing the overall stereochemical
outcome of the above reactions is proposed in Figure 3, with
a distinction made between silanes 3 and 4 and silane 7. Con-
sidering the stereochemistry of silylium ions and that of the
starting silanes (Schemes 1 and 2), it is assumed that hydrogen
abstraction by the trityl cation occurs with inversion of config-
uration at the silicon center. For instance, Ib should provide si-
lylium ion Ib+ with inversion, which upon addition of LiAlH4,
again with inversion, would return silane Ib in an overall ste-
reoretentive process. Epimerization of the silicon center may
occur upon warming (Ib+!Ia+, for example, 4b) or addition of
a chelating ether (Ib+!IIb+, for example, 3b),[9] thus relieving
the steric interactions between the Lewis basic group and the
Large (L) substituent in Ib+. Addition of ether (solv.) likely
occurs with inversion (inv.), with disruption of the interaction
between Si+ and LB (Ib+!IIb+). Equilibration of IIb+ into IIa+
may then proceed through the displacement of the ether sol-
vent in IIb+ by a second molecule of ether (IIb+!III!IIa+),
this route proceeding through a double inversion of configura-
tion at silicon and a putative pentacoordinated species III. In-
tramolecular displacement of the solvent ligand in IIa+, with
inversion, followed by reduction of the more stable Ia+ (lack-
ing the LB$L interaction), again with inversion, would finally
lead to the silane Ia.
À78/20
À78/20
À78/20
1.2 equiv
2 equiv
3 equiv
[a] The starting silane in CD2Cl2 was treated with 1 equiv of Ph3C+
[B(C6F5)4]À and the reaction mixture stirred at the indicated temperature
before being treated with LiAlH4 at À788C. [b] Diastereomeric ratio esti-
mated by 1H NMR spectroscopy. [c] Quant. yield. [d] 76% Yield. [e] 97%
Yield. [f] 77% Yield.
tion had occurred under these conditions (see below). Genera-
tion of 7b+ at À788C in the absence of ether, followed by the
addition of LiAlH4, returned silane 7b in quantitative yield with
no loss of stereochemical integrity (Table 2, entry 1). When the
experiment was repeated with a 60:40 mixture of 7a/7b, com-
plete epimerization was observed at À788C (entry 2, Table 2),
showing that the putative p-interaction between the aryl sub-
stituent and the cationic silicon center was relatively weak
compared to that between the oxygen lone pair and the silyli-
um center, resulting in a loss of the chiral memory at silicon.
Finally, epimerization of the silicon center in diastereomeri-
cally pure 7b was conducted as above for 3 and 4, using the
standard hydride transfer protocol with various amounts of
ether, before addition of LiAlH4. As summarized in Table 2, con-
trasting results are obtained depending on the quantity of
added ether. Addition of 1.2 equiv of ether (entry 3, Table 2),
followed by 1 equiv of the hydride led to epimerization in
favor of the isomer 7a.[17] In turn, addition of 2 or 3 equiv of di-
ethyl ether, under the same conditions (T8C) led back to 7b as
the major isomer (entries 4 and 5, Table 2), indicating that dis-
tinct epimerization processes operate in systems 3, 4, and 7.
Lastly, it is worth mentioning that this epimerization process
constitutes an easy way to obtain 7a that could not be pre-
This picture well describes observations made for silanes 3
and 4 and follows the relative stability of the corresponding si-
lylium ions Ia+ and Ib+ in equilibrium (Figure 2), when a cata-
lytic amount of ether is added. When a stoichiometric amount
(or more) of ether is used, as for silane 7b, silyloxonium ions
IIa+ and IIb+, rather than free ions Ia+ and Ib+ are present in
the mixture, and reasoning should thus be made on these spe-
cies. Using 1.2 equiv of ether, silane Ib (7b) is converted pre-
dominantly into Ia (7a; entry 3, Table 2) after addition of
Chem. Eur. J. 2015, 21, 11573 – 11578
www.chemeurj.org
11576
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
LiAlH4, which may be rationalized invoking pathway Ib!Ib+!
IIb+. Addition of a stoichiometric amount of ether fully con-
verts silylium Ib+ into IIb+, which upon reaction with LiAlH4
provides Ia with overall inversion of configuration. In this case,
the three elementary steps, including the sequence Ib!Ib+!
IIb+, and the hydride displacement of the ether ligand from
IIb+, proceed with inversion of configuration at the silicon
center. In contrast, when an excess of ether is present in the
medium (entries 4 and 5, Table 2), a rapid equilibrium between
silyloxoniums ions IIa+ and IIb+ (likely through III) probably
takes place, the final diastereomeric ratio thus reflecting the
relative stability of these silyloxonium species, as supported by
DFT calculations (see the Supporting Information).[18]
146.5, 143.3, 142.5, 139.7, 136.2, 134.8, 115.7, 115.5, 72.6, 72.4, 25.5„
24.2, 20.1, 18.9, À6.6, À7.3 ppm; 29Si NMR (60 MHz, CD2Cl2): d=
53.3 (3a+) 52.5 ppm (3b+); 11B NMR (96 MHz, 1,2-DCB d-4): d=
À16.8 ppm; 19F NMR (188 MHz, 1,2-DCB d-4): d=À131.7, À162.2,
À166.0 ppm.
General procedure for the epimerization of silylium ion 3a
with ether
Using a dry Schlenk tube equipped with a magnetic stirrer, tert-
butyl[1-(2-methoxynaphthalen-1-yl)naphthalen-2-yl]methylsilane
3a (1 equiv) and tButyl methyl ether (0.1 equiv) in dry CH2Cl2 were
added together. The solution was cooled to À788C and a solution
of Ph3C+[B(C6F5)4]À (1 equiv) in CH2Cl2 was added. The resulting so-
lution was stirred at À788C for 10 min and, if necessary, warmed
to 208C for 2 h. The solution was cooled to À788C and LiAlH4
(1 equiv) in suspension in Et2O was added. The resulting solution
was stirred for 30 min at À788C. A saturated aqueous solution of
NaHCO3 was added, the organic layer was separated and the aque-
ous layer was extracted twice with CH2Cl2. Combined organic
layers were dried over Na2SO4, filtered and the solvent was concen-
trated under vacuum. The residue was then purified by flash chro-
matography on silica gel (Petroleum ether/CH2Cl2 9:1) to afford si-
lanes 3a,b.
Conclusion
In summary, we reported our preliminary studies on the con-
figurational stability of Lewis base-stabilized silyliums, with dif-
ferent scenarios depending on the reaction conditions. These
outcomes ranged from perfect chiral memory to complete epi-
merization through warming or addition of an external Lewis
basic ether, illustrating in the latter case an unusual transfer of
axial to Si-centered chirality. The interaction between a lone
pair and the silylium center was found to be much stronger
than the corresponding p-interaction as illustrated by the loss
of chiral memory in systems 7. The stereochemistry of the nu-
cleophilic substitution at silicon in chiral silyloxonium ions was
also examined. Chirality on the silicon center in stabilized silyli-
um ions constitutes a useful tool, providing information rela-
tive to the strength of the interaction between the Lewis base
and the cationic silicon center. Experimental evidence support-
ed by computational studies have been made available, which
should serve to design new silylium cations with a pacified
electrophilic silicon center for further use in asymmetric cataly-
sis. Such studies are now underway in our laboratories and will
be reported in due course.
General procedure for the epimerization of silylium ion 7b
with ether
Using a dry Schlenk tube equipped with a magnetic stirrer, tbutyl-
(methyl)[1-(2-phenylnaphthalen-1-yl)naphthalen-2-yl]silane
7b
(d.r.>98:2, 1 equiv) and an appropriate amount (see Table 2) of
freshly distilled diethyl ether in dry CH2Cl2 were added together.
The solution was cooled to À788C and a solution of Ph3C+
[B(C6F5)4]À (1 equiv) in CH2Cl2 was added. The resulting solution
was stirred at À788C for 10 min and, if necessary, warmed to 208C
for 2 h. The solution was cooled to À788C and LiAlH4 (1 equiv) in
suspension in Et2O was added. The resulting solution was stirred
for 30 min at À788C. A saturated aqueous solution of NaHCO3 was
added, the organic layer was separated and the aqueous layer was
extracted twice with CH2Cl2. Combined organic layers were dried
over Na2SO4, filtered, and the solvent was concentrated under
vacuum. The residue was then purified by flash chromatography
on silica gel (petroleum ether/CH2Cl2 9:1) to afford silanes 7a,b.
Experimental Section
General procedure for the preparation of silylium ions
Acknowledgements
In a dry J. Young NMR tube, a solid sample of silane (1 equiv) was
dried under vacuum for 1 h, flushed with argon, and transferred
into a glovebox filled with argon. Deuterated solvent (0.25 mL) was
added, followed by a solution of Ph3C+[B(C6F5)4]À (1 equiv) in the
same deuterated solvent (0.25 mL). The tube was then sealed and
subjected to NMR measurements.
We thank CNRS, MNERT, ANR (Catapult N82011-BS08 01102) for
generous support. We are grateful to N. Pinaud, and J. M. Las-
nier (CESAMO, ISM, University of Bordeaux) for NMR experi-
ments and to Dr. B. Kauffmann and S. Massip (IECB, Bordeaux)
for X-ray diffraction studies.
Tert-butyl(2’-methoxy-[1,1’-binaphthalen]-2-yl)(methyl)silylium
(3a+, 3b+): Following the above general procedure for analysis
of silyliums ions, silylium ions 3a+ and 3b+ were prepared
from tButyl(2’-methoxy-[1,1’-binaphthalen]-2-yl)(methyl)silane 3a,b
(20 mg, 0.052 mmol) and Ph3C+[B(C6F5)4]À (50 mg, 0.052 mmol) in
[D2]dichloromethane. 1H NMR (300 MHz, CD2Cl2): d=8.32–8.22 (m,
2.7H), 8.14–8.08 (m, 2H), 7.97 (d, J=8.2 Hz, 0.5H), 7.92–7.85 (m,
1.7H), 7.80 (d, J=8.1 Hz, 0.5H), 7.76–7.64 (m, 4.5H), 7.52–7.40 (m,
4.2H), 7.34–7.19 (8.9H), 7.18–7.12 (m, 5.8H), 4.61 (s, 1.4H), 4.57 (s,
1.3H), 1.48 (s, 4.1H), 1.07 (s, 1.6H), 0.75 (s, 4.4H), 0.35 ppm (s,
1.4H); 13C NMR (50 MHz, 1,2-DCB d-4): d=151.3, 148.4, 148.3,
Keywords: cations · chirality · H transfer · silylium ions ·
stereochemistry
[1] For reviews on silylium ions chemistry, see: a) T. Müller, Silylium ions and
stabilized silylium ions in Science of Synthesis, Knowledge updates
Vol. 2013/3, (Ed. M. Oestreich), Thieme, Stuttgart, 2013, pp. 1; b) H. F. T.
Klare, M. Oestreich, Dalton Trans. 2010, 39, 9176; c) A. Sekiguchi, V. Y.
Lee, Organometallic compounds of low-coordinate Si, Ge, Sn and Pb,
Wiley, Hoboken, 2010.
Chem. Eur. J. 2015, 21, 11573 – 11578
www.chemeurj.org
11577
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[2] K.-C. Kim, C. A. Reed, D. W. Elliott, L. J. Mueller, F. Tham, L. Lin, J. B. Lam-
bert, Science 2002, 297, 825.
16579; h) M. Reißmann, A. Schäfer, R. Panisch, M. Schmidtmann, M.
Bolte, T. Müller, Inorg. Chem. 2015, 54, 2393.
[3] a) K. Hara, R. Akiyama, M. Sawamura, Org. Lett. 2005, 7, 5621; b) R. K.
Schmidt, K. Müther, C. Mück-Lichtenfeld, S. Grimme, M. Oestreich, J. Am.
Chem. Soc. 2012, 134, 4421.
[9] For a recent report on intramolecularly sulfur-stabilized chiral silicon
cations, see: V. H. G. Rohde, P. Pommerening, H. F. T. Klare, M. Oestreich,
Organometallics 2014, 33, 3618.
[4] a) V. J. Scott, C¸.-C¸. Remle, O. V. Ozerov, J. Am. Chem. Soc. 2005, 127,
2852; b) C. Douvris, O. V. Ozerov, Science 2008, 321, 1188; c) C. Douvris,
C. M. Nagaraja, C.-H. Chen, B. M. Foxman, O. V. Ozerov, J. Am. Chem. Soc.
2010, 132, 4946; d) R. Panisch, M. Bolte, T. Müller, J. Am. Chem. Soc.
2006, 128, 9676; e) N. Lühmann, H. Hirao, S. Shaik, T. Müller, Organome-
tallics 2011, 30, 4087; f) N. Lühmann, R. Panisch, T. Müller, Appl. Organo-
met. Chem. 2010, 24, 533; g) O. Allemann, S. Duttwyler, K. K. Baldridge,
J. S. Siegel, Science 2011, 332, 574.
[5] a) A. Schäfer, M. Reißmann, A. Schäfer, W. Saak, D. Haase, T. Müller,
Angew. Chem. Int. Ed. Angew. Chem. Int. Ed. Engl. 2011, 50, 12636;
Angew. Chem. 2011, 123, 12845; b) M. Reißmann, A. Schäfer, S. Jung, T.
Müller, Organometallics 2013, 32, 6736.
[10] a) P. D. Bartlett, F. E. Condon, A. Schneider, J. Am. Chem. Soc. 1944, 66,
1531; b) J. Y. Corey, J. Am. Chem. Soc. 1975, 97, 3237.
[11] a) R. J. P. Corriu, C. GuØrin, J. J. E. Moreau in Topics in Stereochemistry
Vol. 15, (Ed. E. L. Eliel), Wiley, New York, 1984; pp. 43; b) S. Varaprath,
D. H. Stutts, J. Organomet. Chem. 2007, 692, 1892.
[12] G. A. Olah, X.-Y. Li, Q. Wang, G. Rasul, G. K. S. Prakash, J. Am. Chem. Soc.
1995, 117, 8962.
[13] Z. Tang, B. Mathieu, B. Tinant, G. Dive, L. Ghosez, Tetrahedron 2007, 63,
8449.
[14] A 2:98 ratio is also predicted by calculations at the DFT level (M06–2X/
cc-pvtz) with 2.2 kcalmolÀ1 difference found for 7a/7b.
[15] Calculation of 29Si NMR chemical shifts at the DFT level led to silylium
7a+/7b+ values of 83.9 and 89.6 ppm respectively (see the Supporting
Information), in good agreement with Siegel’s results on closely related
systems (see ref. [7]).
[6] M. Johannsen, K. A. Jorgensen, G. Helmchen, J. Am. Chem. Soc. 1998,
120, 7637.
[7] a) S. Duttwyler, Q.-Q. Do, A. Linden, K. K. Baldridge, J. S. Siegel, Angew.
Chem. Int. Ed. 2008, 47, 1719; Angew. Chem. 2008, 120, 1743; b) P. Ro-
manato, S. Duttwyler, A. Linden, K. K. Baldridge, J. S. Siegel, J. Am.
Chem. Soc. 2010, 132, 7828; c) P. Romanato, S. Duttwyler, A. Linden,
K. K. Baldridge, J. S. Siegel, J. Am. Chem. Soc. 2011, 133, 11844.
[8] a) M. Chauhan, C. Chuit, R. J. P. Corriu, A. Mehdi, C. ReyØ, Organometal-
lics 1996, 15, 4326; b) T. Müller, Angew. Chem. Int. Ed. 2001, 40, 3033;
Angew. Chem. 2001, 113, 3123; c) A. Y. Khalimon, Z. H. Lin, R. Simiones-
cu, S. F. Vyboishchikov, G. I. Nikonov, Angew. Chem. Int. Ed. 2007, 46,
4530; Angew. Chem. 2007, 119, 4614; d) M. Lehmann, A. Schulz, A. Villin-
ger, Angew. Chem. Int. Ed. 2009, 48, 7444; Angew. Chem. 2009, 121,
7580; e) K. Müther, R. Frçhlich, C. Mück-Lichtenfeld, S. Grimme, M. Oest-
reich, J. Am. Chem. Soc. 2011, 133, 12442; f) M. F. Ibad, P. Langer, A.
Schulz, A. Villinger, J. Am. Chem. Soc. 2011, 133, 21016; g) K. Müther, P.
Hrobarik, V. Hrobarikova, M. Kaupp, M. Oestreich, Chem. Eur. J. 2013, 19,
[16] Diethyl ether was used instead of tBuOMe, in order to avoid the pres-
ence of an additional stereogenic oxygen center.
[17] It must be pointed out that strict addition of 1 equiv of ether led to
low yield of 7a/7b (in favor of 7a). In turn, a larger amount (1.5 equiv)
led to equilibration in favor of 7b, as shown in entries 4 and 5 (Table 2).
[18] Calculations at the DFT level (M06–2X/cc-pvtz) predict that silyloxonium
IIb+ is more stable than IIa+ by 2.1 kcalmolÀ1 (97:3) in Et2O and by
1.1 kcalmolÀ1 (85:15) in Me2O. The formation of 7b as a major isomer
might thus be explained by the displacement of the solvent molecule
in IIa+ by LiAlH4 (with inversion) under a Curtin–Hammett regime.
Received: May 20, 2015
Published online on July 2, 2015
Chem. Eur. J. 2015, 21, 11573 – 11578
www.chemeurj.org
11578
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim