Stereocontrol in Nucleophilic Substitution Reactions at Silicon: The Role of Permutation in Generating Silicon-Centered Chirality

Article abstract of DOI:10.1021/jacs.5b00861

Intramolecular isomerization in pentacoordinate compounds can play an essential role for the adjustment of defined stereochemical information. Here, we present a conclusive mechanism of a stereocontrolled reaction on chiral dimethoxysilanes that opens new aspects in understanding the origin of creating silicon-centered chirality during a nucleophilic substitution process. By combining experimental, structural, and quantum chemical methods, we were able to disclose an interconversion process, based on consecutive Berry-type motions, as the most plausible mechanism for describing the stereochemical outcome in suchlike substitution reactions.

Full text of DOI:10.1021/jacs.5b00861

Communication
pubs.acs.org/JACS
Stereocontrol in Nucleophilic Substitution Reactions at Silicon: The
Role of Permutation in Generating Silicon-Centered Chirality
Jonathan O. Bauer*,† and Carsten Strohmann*
Anorganische Chemie, Technische Universitat Dortmund, Otto-Hahn-Straße 6, 44227 Dortmund, Germany
̈
S
* Supporting Information
stereoconvergency, and studies concerning the configurational
ABSTRACT: Intramolecular isomerization in pentacoor-
dinate compounds can play an essential role for the
adjustment of defined stereochemical information. Here,
we present a conclusive mechanism of a stereocontrolled
reaction on chiral dimethoxysilanes that opens new aspects
in understanding the origin of creating silicon-centered
chirality during a nucleophilic substitution process. By
combining experimental, structural, and quantum chemical
methods, we were able to disclose an interconversion
process, based on consecutive Berry-type motions, as the
most plausible mechanism for describing the stereo-
chemical outcome in suchlike substitution reactions.
stability of the substitution products gave first hints that the
substitutions possibly follow a kinetically controlled, multistep
mechanism.8 Owing to the lack of asymmetric methods that
apply nucleophilic substitution reactions for providing silicon-
stereogenic molecules,8,9 the mechanisms of transferring chiral
information onto a silicon center are thus not really understood
until now.
Therefore, in our present contribution, we examine in detail
the intramolecular transfer of chirality in the course of a
nucleophilic substitution reaction at a prochiral silicon center.
Detailed mechanistic investigations have provided insight into
the origin of creating a defined configuration at silicon. First
structural data of a unique, inert precoordination compound
have supported our drawn mechanistic picture of stereo-
differentiation between two diastereotopic methoxy groups.
For our studies, we focused on a reaction system consisting of
(trimethylsilyl)methyllithium and the cyclohexyldimethoxysi-
lane (SC)-1 that was easily synthesized by our previously
established procedure10 from cyclohexyltrimethoxysilane and
the lithium salt of (2S)-2-(methoxymethyl)pyrrolidine (SMP) in
94% yield (Table 1). Except for the SMP moiety and the silicon-
bound methoxy groups, there are no additional coordination
sites for an attacking nucleophile in the periphery of the substrate
backbone of (SC)-1. After the reaction had been first carried out
in diethyl ether during slowly warming the mixture from −80 °C
to room temperature, (SC)-1 was converted into (SC,RSi)-2 in
70% yield and with excellent stereoselectivity (d.r. = 98:2) (Table
1, Case I).11 Astonishingly, the reaction proceeded with a distinct
decrease in stereocontrol when performing in pentane under
otherwise same reaction conditions. Now, the diastereomeric
ratio only amounts to 70:30 (Table 1, Case II). It is worth
mentioning that we even noticed a slight shift of the
diastereomeric ratio (d.r. = 45:55) in favor of the SSi-configured
substitution product 2 after carrying out the reaction in pentane
at 0 °C (Table 1, Case III). In another experiment in pentane
solution, the reaction mixture had first been stirred for 48 h at
−30 °C, before it was slowly warmed to room temperature
afterward (Table 1, Case IV). Also in this case, (SC,RSi)-2 was
obtained with only moderate stereochemical purity (d.r. =
74:26). In each case, the optical purity of compound 2 was
verified by 1H, 13C, and 29Si NMR spectroscopy. Already at this
point, our investigation concerning the reactivity and selectivity
suggests that the stereodetermining step needs to differentiate
between activation barriers that differ only minimally in energy.
entacoordinate species have long been known as central
P
intermediates in nucleophilic substitution reactions at
silicon.1 In case of substitutions at stereogenic centers, the
stereochemical outcome of these reactions can strongly depend
on the kinetics of intramolecular permutation that may occur in
intermediate, higher-coordinate compounds.2 In addition, also in
transition-metal-mediated reactions, isomerization processes can
play an important role.3 This was impressively shown by Schrock
and Hoveyda in asymmetric catalysis, where the dynamic
behavior of molybdenum-based catalysts with metal-centered
chirality is responsible for an exceptional efficiency and selectivity
in homogeneous, enantioselective alkene metathesis.4 In a
profound, recently performed theoretical investigation, Couzijn
et al. have demonstrated that the long-known Berry pseudor-
otation (BPR)5 can be interpreted as the elemental mechanism
for describing configurational stereomutations in pentacoordi-
nate, trigonal-bipyramidal species (Figure 1).6
Figure 1. BPR as fundamental process for configurational changes in
trigonal-bipyramidal compounds.5 Axial substituents: blue; equatorial
substituents: red.
In the meanwhile, the interest in synthesis and application of
silicon-chiral silanes is continually growing, as can be seen on the
great number of publications during the past few years.7 Just a
short while ago, we reported on a highly stereocontrolled
reaction of chiral aminodimethoxysilanes with organolithium
reagents.8 Remarkable observations, such as the occurrence of
Received: January 26, 2015
Published: March 24, 2015
© 2015 American Chemical Society
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complex (cyclohexyl group and coordinated nucleophile are
situated at the same site) that passes through the energetically
lowest transition state [TS1]⧧ with an activation barrier of 45 kJ
mol−1, leading to the trigonal-bipyramidal key-intermediate I1.15
In this pentacoordinate silicon species, the lithium cation is
chelated by three donor atoms. As a result of this coordination,
the nitrogen atom directly attached to silicon becomes also
stereogenic with a fixed configuration. Another imaginable
attack16 at the silicon center out of an alternative anti-configured
precomplex (anti-Cy-E) via the transition state [TS2]⧧ can
unequivocally be excluded, considering the difference (ΔΔE⧧)
between [TS1]⧧ and [TS2]⧧ of 28 kJ mol−1 (Figure 2). The
energetic preference of [TS1]⧧ toward [TS2]⧧ can be ascribed to
a stereoelectronic effect (better HOMO|LUMO overlap in
[TS1]⧧); due to the quasi axial location of the silicon-bound
OMe group in [TS1]⧧, the extra electron density of the incoming
carbanion might be better stabilized across the molecular
framework.17 The formation of I1 through the energetically
lowest barrier [TS1]⧧ marks the first critical point for the
stereochemical course of the reaction.
However, the question remains, in which step the final
determination of the stereochemistry of the substitution product
actually occurs. In case of an elimination of the methoxy group
coordinated to lithium out of the pentacoordinate intermediate
I2 in which [BPR1]⧧ had been turned the leaving OMe group in
the axial position, the substitution product would feature the SSi
configuration (Figure 3, Path A). Hence, a direct liberation of the
methoxy group as the predominant path appears unlikely.
Instead, by the action of two Berry-type pseudorotations, I1 can
be interconverted via I3 into the stereoelectronically most
favored intermediate I4, in which the two Si−OMe groups are
located in the axial position.17 From that point, the liberation of
the methoxy group (I4 → P2) would terminate the substitution
process under formation of the RSi-configured product (Figure 3,
Path B). In this mechanistic picture, the stereochemistry of the
final product depends on a competition between the two
diastereomorphic transition states [BPR1]⧧ and [BPR2]⧧
(ΔΔE⧧ = 10 kJ mol−1) belonging to Berry-type motions (Figure
3). The energetic preference of I3 toward I2 might be
predominantly the result of a reduced repulsive interaction
between the cyclohexyl and the pyrrolidine ring in I3, which is
also expressed in the respective transition state [BPR2]⧧ (for
structural details, see the Supporting Information). In addition to
this steric effect, the movement of the electronegative pyrrolidine
moiety into the quasi axial position through [BPR2]⧧ might also
be taken into account for explaining the lower activation barrier
for the conversion I1 → I3.
Table 1. Stereocontrolled Synthesis of (SC,RSi)-2 and
Determination of the Absolute Configuration at the
Stereogenic Silicon Center
c
case
solvent
temperature
d.r.
a
I
diethyl ether
pentane
−80 °C → rt (20 h)
−80 °C → rt (20 h)
0 °C (15 min) → rt (20 h)
−80 °C → −30 °C (48 h) → rt
98:2
b
II
70:30
45:55
b
III
pentane
b
IV
pentane
74:26
a
b
c
70% yield of (SC,RSi)-2. Yields have not been determined. The
diastereomeric ratio of 2 has been determined by integration of
baseline-separated 1H NMR signals belonging to the two diaster-
eomers.
In addition, the coordinating properties of diethyl ether seem to
have a positive influence on the stereochemical outcome. The
absolute configuration at the stereogenic silicon center could
unambiguously be determined as RSi by single-crystal X-ray
diffractional analysis of the dimeric, α-lithiated compound
[(SC,SC,RSi)-3]2·C5H12 (Table 1).12 The structural features are
discussed in more detail in the Supporting Information.
In a next purpose, the initial nucleophilic attack at the silicon
center of the reaction shown in Table 1 was calculated [M052X/
6-31+G(d)].13,14 Assuming that monomeric complexes are the
acting species in the substitution mechanism, the monomeric,
cyclohexyl-substituted precomplex syn-Cy-E was chosen as a
plausible starting structure. In this way, the first and likewise rate-
determining step of the reaction could be formulated (Figure 2).
It is in fact a syn-type configuration of the initially formed
Altogether, the impact of thermal variations on the
diastereomeric ratio of the final product (Table 1, Cases II and
III) should be the greater, the smaller the energy differences
between stereodetermining transition states are. This corre-
sponds nicely with our calculated permutational process.18 The
higher diastereoselectivity in Case 1 of Table 1 may be a
consequence of additional solvent coordination to the lithium
center, which could lead to a greater energetic difference between
competing transition states.
Given our established experience in the field of structural
elucidation of prelithiation complexes,19 we attempted the
isolation of an unreactive complex, comprising an amino-
dimethoxysilane and an alkyllithium reagent, in order to give
our proposed starting structures more significance on an
experimental basis. Silanes bearing both silicon−nitrogen and
silicon−oxygen bonds are suitable ligands, with the Si−OMe
Figure 2. Activation barriers of the axial nucleophilic attack at silicon,16
starting from the cyclohexyl-substituted reactants [M052X/6-31+G-
(d)].13 Numerical values in [kJ mol−1]. Cy = cyclohexyl.
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Communication
Figure 3. Stereodetermining differentiation between the two diastereomorphic permutational transition states [BPR1]⧧ and [BPR2]⧧ [M052X/6-
31+G(d)].13 Numerical values in [kJ mol−1]. Cy = cyclohexyl. P1 = [(SC,SSi)-2·LiOMe], P2 = [(SC,RSi)-2·LiOMe].
group serving as an effective coordinating group for alkyllithum
reagents.20 To the best of our knowledge, presubstitution
complexes that are formed prior to nucleophilic attack at silicon
have not been observed nor experimentally characterized to
date.9
The chance to form an inert complex of (trimethylsilyl)-
methyllithium21 and (SC)-1 seemed unpromising since the
reaction already starts at low temperatures. However, the related
tert-butyl-substituted compound (SC)-4 has been proved to be
far less reactive concerning a nucleophilic substitution in our
previous studies yet.8 In fact, after we had exchanged the
cyclohexyl by a tert-butyl group, the reactivity of the amino-
dimethoxysilane toward a nucleophilic attack was diminished to
the point that we finally succeded in isolating an unreactive,
dimeric adduct of (SC)-4 and Me3SiCH2Li at −70 °C. This
complex could be fully characterized by means of X-ray
crystallography and NMR spectroscopy (Scheme 1 and Figure
4).
Figure 4. Molecular structure of [(SC)-4·Me3SiCH2Li]2 in the crystal.12
The hydrogen atoms are omitted for clarity (except the hydrogen atoms
bound to the metalated carbon atoms). Selected bond lengths [Å] and
angles [°]: C5−O2 1.428(3), C6−O1 1.446(3), C13−Li 2.199(5),
C13−Li′ 2.365(4), Li−O3 2.017(4), Li−O1 2.073(4), Li−C13′
2.365(4), Li−Li′ 2.574(7), N−Si1 1.694(2), O1−Si1 1.6521(15),
O2−Si1 1.6407(16), O3−Li−O1 96.28(17), Li−C13−Li′ 68.55(17),
C13−Li−C13′ 107.38(16), C6−O1−Si1 120.95(14), C5−O2−Si1
122.98(16).
Scheme 1. Formation of the Inert Precomplex [(SC)-4·
Me3SiCH2Li]2
low molecular complexes with participation of the SMP OMe
and one Si−OMe function.
We have also calculated the respective transition states of a
nucleophilic attack at silicon out of the monomeric, tert-butyl-
substituted complexes syn-tBu-E and anti-tBu-E. Again, the axial
attack via the syn-configured transition state turned out to be the
energetically most favored one (ΔΔE⧧ = 36 kJ mol−1). The
remarkably higher activation barriers, compared to the cyclo-
hexylsilane, reflect the observed inhibition of the reaction of
(SC)-4 with Me3SiCH2Li (for details, see the Supporting
Information).
In conclusion, we have shown for stereocontrolled nucleo-
philic substitution reactions on chiral aminodimethoxysilanes
that stereomutation is of fundamental importance for creating
silicon-centered chirality. The results of these studies might not
only be of interest for the understanding of stereocontrolled
reactions in heavy main group element chemistry; they might
also bring light into transition-metal-mediated reactions in which
high-coordinate intermediates play a crucial role.
The dimer22 [(SC)-4·Me3SiCH2Li]2 crystallized in the form of
colorless blocks in the tetragonal crystal system, space group
P41212 (Figure 4).12 The lithium center is coordinated solely by
the methoxymethyl side arm and the silicon-bound, pro-S
methoxy group. The tert-butyl group shows a syn-type
configuration with respect to complexed Me3SiCH2Li. The
Li−O1 bond length [2.073(4) Å] is only slightly elongated,
1
compared to Li−O3 [2.017(4) Å]. The H NMR shifts ([d6]-
benzene) of the Si−OMe groups of the free compound (SC)-4
are at 3.41 and 3.47 ppm.8 In [(SC)-4·Me3SiCH2Li]2, one of the
1
two silicon-bound methoxy groups shows a H NMR signal at
higher field (3.35 ppm), with the other Si−OMe group almost
ASSOCIATED CONTENT
7
■
remaining unchanged (3.50 ppm). The Li NMR spectrum
S
* Supporting Information
shows a singlet at 2.44 ppm. Also in solution, we can therefore
assume the existence of a well-defined species, in which only one
of the two diastereotopic Si−OMe groups is involved in
coordination to the lithium center. Moreover, this structure
supports our proposed mechanism of a nucleophilic attack out of
Experimental procedures, characterization data, detailed struc-
tural information, quantum chemical calculations, and complete
ref 13. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*jonathan.bauer@weizmann.ac.il
*mail@carsten-strohmann.de
■
Present Address
†Department of Organic Chemistry, The Weizmann Institute of
Science, P.O. Box 26, 76100 Rehovot, Israel
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Deutsche Forschungsgemeinschaft (DFG) for
financial support. J.O.B. thanks the Fonds der Chemischen
Industrie (FCI) for a Chemiefonds Fellowship and the Elite
Network of Bavaria for scientific mentoring programs.
(10) Bauer, J. O.; Strohmann, C. Chem. Commun. 2012, 48, 7212.
(11) The inverse reactivity (substitution of the Si−N function) is true
under acidic conditions, e.g., with alcohols and silanols. See: refs 8. and
10.
(12) CCDC 1019420 [(SC,SC,RSi)-3]2·C5H12 and CCDC 1019421
[(SC)-4·Me3SiCH2Li]2 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. For details, see also the Supporting Information.
(13) Frisch, M. J. et al., Gaussian 03, revision E.01; Gaussian, Inc.:
Wallingford, CT, 2004. For details, see the Supporting Information.
(14) For a profound comparison of different methods for calculations
with alkyllithium species, see: Gessner, V. H.; Koller, S. G.; Strohmann,
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