Synthesis of silaoxazolinium salts bearing weakly coordinating anions: Structures and catalytic activities in the aldol reaction

Article abstract of DOI:10.1021/om400017f

The synthesis and structures of silaoxazolinium salts 2 and their application to the catalytic Mukaiyama aldol reaction are described. The reaction of (N-amidomethyl)dimethylchlorosilane (1a) or (N-amidomethyl) bis(trimethylsilyl)chlorosilane (1b) with me

Full text of DOI:10.1021/om400017f

Article
pubs.acs.org/Organometallics
Synthesis of Silaoxazolinium Salts Bearing Weakly Coordinating
Anions: Structures and Catalytic Activities in the Aldol Reaction
Anugu Chandra Sheker Reddy,† Zhang Chen,† Tohru Hatanaka,‡ Tatsuya Minami,‡
and Yasuo Hatanaka*,‡
†National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, Tsukuba, Ibaraki, 305-8565, Japan
‡Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Osaka 558-8585, Japan
S
* Supporting Information
ABSTRACT: The synthesis and structures of silaoxazolinium salts 2 and their application to
the catalytic Mukaiyama aldol reaction are described. The reaction of (N-amidomethyl)-
dimethylchlorosilane (1a) or (N-amidomethyl)bis(trimethylsilyl)chlorosilane (1b) with
metal salts of weakly coordinating anions such as Na[TFPB] (TFPB = B[3,5-
−
−
(CF3)2C6H3]4 ) and Cs[CB11H12] (CB11H12 = carba-closo-dodecaborate) gave the
corresponding five-membered-ring silaoxazolinium salts 2 in high yields (93−97%). The
structures of a series of silaoxazolinium salts 2 were determined by X-ray crystal analysis as well as 29Si NMR spectra. It was
proved that the silicon atoms of silaoxazolinium salts 2a,b are nearly completely free from the coordination of anions, and the
geometries of the silicon centers are distorted tetrahedra. The 29Si chemical shifts of salts 2 appeared in the range +31 to +49
ppm, revealing the appreciable silylium cation character of the silicon. The silaoxazolinium salt of TFPB anion 2a exhibited
effective catalytic activity for the Mukaiyama aldol reaction of low reactive unactivated ketones such as cyclohexanone and
acetophenone, giving the corresponding aldol products in high yields. The catalytic activity is highly dependent on the nature of
the counteranions and the substituents on the silicon. DFT calculations of silaoxazolinium cation have disclosed that positive
charges of silaoxazolinium cations are mainly located at the silicon atoms, while the nitrogen atoms and oxygen atoms are
negatively charged.
Scheme 1
INTRODUCTION
■
The synthesis of highly reactive tetracoodinate silylium cations
[R3SiL]+ (L = coordinating solvent or donor atoms) and
trivalent silylium cations is currently one of the most
challenging topics in the fields of inorganic and organometallic
chemistry.1 Tetracoordinate silylium cations have also attracted
much attention from synthetic chemists, because extremely
electrophilic silylium cations can promote a number of unique
reactions as reagents.2 Moreover, tetracoordinate silylium
cations exhibit effective catalytic activity for organic reactions
such as Diels−Alder reactions and Friedel−Crafts alkylations.3
In contrast, very little attention has been paid to the
chemistry of five-membered-ring cationic silicon compounds
such as silaoxazolinium salts 2 and (N-amidomethyl)dimethyl-
(X)silanes 3−6 (X = I, TfO), although these compounds can
be easily prepared from commercially available materials
(Scheme 1). The synthesis and the crystal structures of five-
membered-ring (N-amidomethyl)dimethyl(X)silanes 3−6 have
been reported in the mechanistic studies of nucleophilic
substitution at a pentacoordinate silicon atom (Scheme 1).4
For compounds 3−6, the positive charge has been thought to
be distributed among oxygen and nitrogen, but most of the
positive charge is located around nitrogen.4a,c,e,f The silicon
atoms of (N-amidomethyl)dimethyl(X)silanes 3−6 are coordi-
nated by iodide or triflate, and the geometries of the silicon
centers gradually vary from trigonal bipyramidal (TBP) to
distorted tetrahedral with weakening coordination of anions.4
Accordingly, completely coordination-free (N-amidomethyl)-
dimethyl(X)silanes are regarded to be silaoxazolinium salts 2. It
seems reasonable to assume that the geometry of the silicon as
well as the extent of X→Si coordination would strongly
influence the charge distribution, the electronic properties of
silaoxazolinium salts, and the Lewis acidity of the silicon.1j,k,5
We have carried out DFT calculations to shed light on the
factors which influence the charge distribution of silaoxazoli-
nium cations. Our calculations have strongly suggested that the
positive charge of silaoxazolinium cations 2 is mainly located at
the silicon, indicating an appreciable Lewis acidity of the silicon.
We report herein the synthesis and X-ray crystal structures of
silaoxazolinium salts 2 having weakly coordinating anions such
−
as B[3,5-(CF3)2C6H3]− (TFPB anion) and CB11H12 (carba-
4
closo-dodecaborate anion) (Scheme 2).6a 29Si NMR spectros-
Received: January 9, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om400017f | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
copy demonstrated that the tetramethylsilaoxazolinium salt of
TFPB 2a and tetramethylsilaoxazolinium salt of CB11H12 2b are
almost free from the coordination of anions in solution.
Furthermore, we have found silaoxazolinium salt 2a to be a
highly effective catalyst for the Mukaiyama aldol reaction of low
reactive unactivated ketones such as cyclohexanone and
acetophenone.3b A detailed comparison of the structures of
silaoxazolinium salts 2 with those of (N-amidomethyl)-
dimethyl(X)silanes 3−6 revealed that the counteranions and
the substituents on the silicon play a decisive role in the
catalytic activity for the Mukaiyama aldol reaction.
Scheme 3
RESULTS AND DISCUSSION
■
Synthesis of Silaoxazolinium Salts 2. Chloride abstrac-
tion from pentacoordinate chlorosilanes 17 by sodium or
cesium salts of weakly coordinating anions such as tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate (TFPB anion) and
−
CB11H12 (1-carba-closo-dodecaborate anion) proceeded
smoothly in dichloromethane, giving tetracoordinate silaox-
azolinium salts 2 in nearly quantitative yields (Scheme 2). A
Scheme 2
the silicon; the ΔSi value indicates the displacement of the
silicon atoms toward the oxygen atoms. Thus, ΔSi increases
with increasing tetrahedral character of the silicon centers.
For a comparison with the structures of 2, the Si−O bond
lengths and the Si···X (X = I, OTf) atomic distances of
representative (N-amidomethyl)dimethyl(X)silanes 3,4e 4,4a
5,4d and 6,4d whose silicon centers are relatively close to
tetrahedral geometry among a number of (N-amidomethyl)-
dimethyl(X)silanes, are shown in Scheme 4.
X-ray Crystal Structures of 2a,b. The crystals of 2a
consist of asymmetric cell a containing crystallographically two
independent structures, 2a(A) and 2a(B). The X-ray crystal
structure of 2a(A) is shown in Figure 1, and the key structural
data of 2a(A) and 2a(B) are shown in Table 1. The structural
series of pentacoordinate chlorosilanes 1 can be easily prepared
from commercially available N-methyl-N-(trimethylsilyl)-
acetamide and (chloromethyl)chlorosilanes R2(ClCH2)SiCl
(1a, R = Me; 1b, R = SiMe3). The resulting silaoxazolinium
salts 2 are colorless crystals and fairly stable in air at room
temperature. Layering the solution of 2 in dichloromethane
with hexane or pentane yielded crystals suitable for X-ray
diffraction. The structural determination by X-ray crystallog-
raphy revealed that the electronic nature and steric bulk of the
substituents on the silicon exert a strong influence on the
geometries of the silicon centers. The key structural parameters
of 2a−d are given in Table 1.
Scheme 4. Structural Data of Representative (N-
Amidomethyl)dimethyl(X)silanes (X = I, OTf)
Silaoxazolinium salts 2 are structurally similar to (N-
amidomethyl)dimethyl(X)silanes (X = I, TfO), which differ
in the extent of the coordination of the anion to the silicon.
The structural changes from pentacoordinate (N-
amidomethyl)dimethyl(X)silanes to tetracoordinate silaoxazo-
linium salts are shown by several structural parameters, as
summarized in Scheme 3. For example, the O−Si bond lengths
of these silicon compounds reflect faithfully the geometries of
the silicon.8 The ratio of Si···X distances to the sum of van der
Waals radii, Si···X/∑vdW, displays the extent of coordination
of the anion to the silicon. When Si···X/∑vdW is larger than
1.0, the silicon center is completely free from the coordination
of the anion. In Scheme 3, ΔSi shows the perpendicular
distance of the silicon center from the planes formed by two
substituents R on the silicon and the carbon atom bonded to
B
dx.doi.org/10.1021/om400017f | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
C4A−Si1A−C3A, C3A--Si1A−C2A, and C4A−Si1A−C2A
bond angles of 2a(A) is 347.3(7)° and that of 2a(B) is
345.9(3)°, which are noticeably smaller than the sum of the
three angles for a regular TBP geometry (360°) (Table 1). In
addition, the out-of-plane distances of the central silicon atom
from the C2A−C3A−C4A plane (∑Si) of 2a are fairly large
(0.3892 Å for 2a(A), 0.3897 Å for 2a(B)) in comparison to
those of 3−6, agreeing with the tetrahedral geometry of the
silicon of 2a. Thus, the weak coordination of the TFPB anion
apparently leads to the tetrahedral geometry of the silicon of 2a.
The X-ray crystallographical analysis of 2b (Figure 2) having
a CB11H12 anion revealed that the shortest distance between
Figure 1. ORTEP representation of 2a. Hydrogen atoms are omitted
for clarity. Thermal ellipsoids are drawn at the 35% probability level.
parameters of 2a(A), including bond lengths, atomic distances,
and bond angles, are nearly identical with those of 2a(B).
Table 1. Key Bond Lengths (Å) and Bond Angles (deg) of
2a−d in X-ray Crystal Structures
2a
2b
2c
2d
Figure 2. ORTEP representation of 2b. Hydrogen atoms are omitted
for clarity. Thermal ellipsoids are drawn at the 35% probability level.
O−Si
1.723(3) (A)
1.721(3) (B)
3.365 (A)
1.734(2)
1.766(5)
1.770(3)
a
−
F−Si
H−Si
C−O
3.353
the silicon atom and the hydrogen atom on CB11H12 (3.175
b
3.175
2.756
Å) is slightly shorter than the sum of the van der Waals radii
(∑vdW for Si···H = 3.20 Å).9 Other structural parameters of
2b are almost identical with those of 2a (Table 1). Thus, the
silicon atom of 2b was proved to be nearly completely free from
the coordination of the anion.
1.292(5) (A)
1.299(5)(B)
89.7(2) (A)
89.7(2) (B)
102.0(3) (A)
102.7(3) (B)
347.3(7) (A)
345.9(3) (B)
0.3892(3) (A)
0.3897(3) (B)
1.308(4)
1.292(7)
88.6(3)
103.2(4)
352.6(3)
0.349(5)
1.310(4)
c
O−Si−C
N−C−Si
90.3(1)
88.6(1)
29Si NMR Spectroscopic Analysis of 2a,b. The 29Si NMR
chemical shifts are highly sensitive to the degree of silylium
cation character, the coordination numbers of silicon, and the
coordination frameworks of silicon.10,11 In general, 29Si
chemical shifts of neutral tetracoordinate silicon compounds
with the coordination framework of SiCCCO (C = alkyl, O =
alkoxy) appear in the range +13 to +19 ppm in CDCl3 and
CD2Cl2.11a As shown in Table 2, δ(29Si) of 2a, which has an
102.0(2)
347.5(5)
0.3826(5)
104.0(2)
352.3(4)
0.3527(3)
d
∑A−Si−B
∑Si
a
The shortest atomic distance between the central silicon atoms and
the fluorine atoms on the TFPB anion. The shortest atomic distance
between the central silicon atoms and the hydrogen atoms on the
CB11H12 anion. Ring carbon. Sum of the three bond angles C−Si−C
(2a,b); sum of the bond angles Si−Si−Si and two C−Si−Si (2c,d).
b
c
d
Table 2. 29Si NMR Chemical Shifts of Silaoxazolinium Salts
a
2a−d
entry
compd
29Si chem shift, ppm
The structural data of (N-amidomethyl)dimethyl(X)silanes
shown in Scheme 4 illustrate that the Si−O bond lengths of 3
(1.831(2) Å), 4 (1.749 Å), 5 (1.75 Å), and 6 (1.745(3) Å) are
longer than that of 2a (1.723(3) Å). The Si−O bond of 2a is
apparently covalent, which is very close to the Si−O covalent
bond length of 1-oxa-3-aza-5-silacyclopentane (1.722 Å),8
proving the tetrahedral geometry of the silicon of 2a (Scheme
3). The Si···I atomic distances of 3 (3.510(2) Å) and 4 (3.734
Å) are significantly shorter than the sum of van der Waals radii
(∑vdW for Si····l = 4.08 Å)9 to indicate that the silicon atoms
are appreciably coordinated by I−. The Si···OTf atomic distances
of 5 (2.79 Å) and 6 (2.762(3) Å) are also considerably shorter
than the sum of van der Waals radii (∑vdW for Si···O = 3.62
Å),9 implying that the silicon atoms are strongly coordinated by
−OTf.
In the case of 2a, the shortest atomic distance between the
silicon atom and the fluorine atom on TFPB− (Si···F = 3.365
Å) is also shorter than the sum of the van der Waals radii
(∑vdW for Si···F = 3.57 Å),9 whereas the Si···F/∑vdW ratio
(0.97) is fairly close to 1.0, indicating that silicon is very weakly
coordinated by the fluorine atom of TFPB−. The geometry of
the silicon atom of 2a is distorted tetrahedral, since the sum of
1
2
3
4
5
2a
46.1
b
2a
−78.4
49.6
2b
2c
2d
−12.0 (terminal Si), 31.0 (central Si)
−13.2 (terminal Si), 27.3 (central Si)
a
b
Measured in CD2Cl2 otherwise noted. Measured in deuterated
THF.
identical coordination framework, appears at +46.1 ppm in
CD2Cl2, which is shifted significantly downfield from those of
the neutral tetracoordinate silicon atoms by ca. +30 ppm (entry
1). It is known that ring strain of cyclic silicon compounds
exerts a pronounced effect on the 29Si chemical shifts; however,
in the case of 1-oxa-2-silacyclic compounds such as 2, this effect
is negligible.11a Thus, the observed downfield shift of δ(29Si) of
2a can be attributed to silylium cation character.5 Bassindale
and Borbaruah have predicted that the 29Si chemical shifts of
coordination-free silaoxazolinium cations will appear at about
+40 ppm in CDCl3 or CD3CN on the basis of experimental
results, strongly supporting our assumption.4c
C
dx.doi.org/10.1021/om400017f | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
When the 29Si NMR spectrum of 2a was measured in
deuterated THF, the 29Si chemical shift of 2a was shifted
markedly upfield to −78.4 ppm (Table 2, entry 2). Generally,
coordination expansion of silicon atoms accompanies upfield
shift of δ(29Si).10,11b The observed upfield shift in THF
indicates the formation of hypervalent silicon species. In
addition, 29Si chemical shifts of cationic pentacoordinate silicon
compounds, whose structures have been unequivocally
determined by spectral methods or X-ray crystallographic
analysis, appear in a wide range from +6.9 to −120 ppm.12 The
large upfield shift of δ(29Si) of 2a (Δδ = −124.5 ppm) observed
in deuterated THF, therefore, can be ascribed to the formation
of cationic pentacoordinate silicon species, formed by
coordination of the THF molecule to the silicon of 2a
([(CD2)4O→SiL4]+). This observation demonstrates the
appreciable Lewis acidity of the silicon of 2a.
flatness of the plane formed by the three silicon atoms and the
C1 atom. Moreover, the Si1−Si2−Si3 bond angle (125.0(1)°)
is very close to that of a regular trigonal bipyramid (120°).
Consequently, the geometry of the central silicon atom of 2c is
ascertained to be TBP. It is reasonable to consider that the two
sterically demanding trimethylsilyl groups of 2c are oriented
toward the equatorial positions in the TBP structure to avoid
steric repulsion; the axial position is occupied by the small
fluorine atom on TFPB−. The structural parameters of
silaoxazolinium salt 2d are nearly identical with those of 2c
(Table 1), indicative of the TBP structure of 2d.
29Si NMR Spectroscopic Analysis of 2c,d. The 29Si NMR
chemical shifts of the central silicon atoms of 2c,d appeared at
+31.0 and +27.3 ppm, respectively, while the terminal silicon
atoms of 2c,d exhibited chemical shifts at −12.3 and −13.2
ppm, respectively (Table 2, entries 4 and 5). In view of the
similar geometric parameters as well as the nearly identical 29Si
chemical shifts of 2c,d, the geometries of the silicon and the
electronic environments around the silicon are affected little by
the counteranions.
The 29Si chemical shift of 2b appeared at +49.6 ppm in
CD2Cl2, which is shifted somewhat downfield in comparison
with 2a, revealing that 2b has the same degree of silylium cation
character as that of 2a (Table 2, entry 3). The similarities of the
geometric parameters as well as the 29Si chemical shifts of 2a,b
demonstrate the extremely weak coordination by anions and
the fairly large degree of silylium cation character.
Crystal Structures of 2c,d. Crystallization of compounds
2c,d from CH2Cl2/hexane yielded colorless crystals suitable for
X-ray crystallographic analysis. The X-ray structures of 2c,d are
shown in Figures 3 and Figure 4, respectively, displaying that
Mukaiyama Aldol Reaction of Poorly Reactive
Ketones Catalyzed by Silaoxazolinium Salts 2. The
usefulness of new silaoxazolinium salts 2 in organic synthesis
can be best appreciated by reference to the limitations in
synthetically important reactions. The effectiveness of 2 as a
Lewis acid catalyst was evaluated by the Mukaiyama aldol
reaction of poorly reactive unactivated ketones such as
acetophenone and cyclohexanone with silyl enol ethers,
which are less reactive than silyl ketene acetals. Numerous
applications of Lewis acid catalysts to the Mukaiyama aldol
reactions of highly reactive activated ketones such as α-keto
esters and α-diketones are known in the literature.13 In
contrast, only a limited number of effective catalysts have been
reported for the Mukaiyama aldol reaction between sterically
congested unactivated ketones and less reactive silyl enol
ethers.3b,13a,14 Unfortunately, these catalysts are highly sensitive
to moisture, and the preparations of the catalysts are tedious.
Silaoxazolinium salt 2a exhibited high catalytic activity for the
Mukaiyama aldol reaction of unactivated ketones with silyl enol
ethers and silyl ketene acetals in CH2Cl2 at room temperature,
giving high yields of the corresponding aldol products. Table 3
summarizes the results of the aldol reaction of unactivated
ketones with silyl enol ethers catalyzed by 2a. The reaction of
sterically congested acetophenone with 1-(trimethylsilyloxy)-
styrene (7a) proceeded smoothly in the presence of 5 mol % of
2a, giving the corresponding β-hydroxy ketone in 82% yield
after acidic workup (entry 1). Moreover, the catalytic
Mukaiyama aldol reaction between the cyclohexanone and 3a
furnished the desired aldol product in 98% yield (entry 2).
Aldol reactions of silyl enol ethers with aliphatic aldehyde and
sterically congested aldehyde were also effectively catalyzed by
2a to give the β-hydroxy ketones in good yields (entries 3 and
4). To avoid the possibility of a proton-promoted reaction, the
reaction of cyclohexanone and 7a was carried out in the
presence of 2a (5 mol %) and the proton scavenger 2,6-di-tert-
butyl-4-methylpyridine (10 mol %). As shown in entry 6, the
addition of proton scavenger did not affect the yield, excluding
the possibility of proton-promoted aldol reaction.
Figure 3. ORTEP representation of 2c. Hydrogen atoms are omitted
for clarity. Thermal ellipsoids are drawn at the 35% probability level.
Figure 4. ORTEP representation of 2d. Hydrogen atoms are omitted
for clarity. Thermal ellipsoids are drawn at the 35% probability level.
two sterically demanding trimethylsilyl groups on the central
silicon exert a considerable influence on the geometries of the
silicon centers. As can be seen from Table 1, the shortest
atomic distance between the silicon of 2c and the fluorine on
TFPB− is 3.353 Å, which is considerably shorter than the sum
of van der Waals radii (∑vdW = 3.57 Å).9 The sum of Si1−
Si2−Si3, Si3−Si2−C1, and Si1−Si2−C1 bond angles of 2c is
352.6°, which is close to 360°, indicating the appreciable
The catalytic activities of silaoxazolinium salts 2 depend
completely upon the substituents on the silicon and the
counteranions. The reaction profile of the Mukaiyama aldol
reaction of cyclohexanone and silyl enol ehter 7a catalyzed by
2a−c is shown in Figure 5. The observed reaction profile
D
dx.doi.org/10.1021/om400017f | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
hexanone and silyl enol ether 7a by using (N-amidomethyl)-
dimethyliodosilanes 4 as a catalyst (5−10 mol %) (Scheme
5).4a Since the structure of compound 4 (e.g., O−Si bond
Table 3. Mukaiyama Aldol Reaction Catalyzed by 2a
Scheme 5
a
b
entry
substrate
aldol (yield, %)
c
1
PhCOMe/3a
8 (82)
9 (98)
10 (96)
c
2
cyclohexanone/3a
Ph(CH2)2CHO/3a
PhCH(Me)CHO/3a
PhCOMe/3b
d
3
d
eg
,
4
11 (81)
12 (92)
9 (96)
d
5
f
6
cyclohexanone/3a
a
Conditions: carbonyl compound (0.60 mmol), 2a (0.03 mmol),
b
c
dichloromethane (2 mL) at room temperature. Isolated. 7a (0.66
mmol) was used. 7a or 7b (1.2 mmol) was used. The syn/anti ratio
of product 11 determined by H NMR spectroscopy was 91/9. The
d
e
length, ∑Si, and Si···I/∑vdW ratio) is close to that of 2a
(Table 1 and Scheme 4), a comparison of catalytic activities
between 4 and 2a in the same reaction gives detailed
information about the counteranion effect on the catalytic
activity. Unexpectedly, compounds 4 failed to catalyze the
reaction under the same reaction conditions (in CH2Cl2, room
temperature for 30 h). An increase in the amount of 4 (10 mol
%) did not improve the yield of the aldol product. An 1H NMR
analysis of the reaction mixture showed that both substrates
and catalyst 4 remained unchanged. Although the reason for
the low catalytic activity of 4 is unclear, the possible explanation
is that the strong coordination of I− to the silicon occupies the
coordination site of the silicon; hence, the cyclohexanone
cannot be activated by coordination to the silicon. This result
strongly suggests that a weakly coordinating anion such as
TFPB− of 2a plays an essential role in promoting the ketone−
aldol reaction.
f
1
reaction was carried out in the presence of 2a (5 mol %) and 2,6-tert-
butyl-4-methylpyridine (10 mol %) in dichloromethane. When the
reaction was carried out at −10 °C, the reaction did not proceed.
g
DFT Calculations. The present work has stemmed from the
DFT calculations of coordination-free silaoxazolinium cation A
(a model compound of silaoxazolinium cations 2c,d) and B (a
model compound of silaoxazolinium cations 2a,b). The MOs
(LUMO and LUMO+1) of A and B calculated at the B3LYP/
6-31G(d) level are illustrated in Figure 6.
Figure 5. Reaction profile of the aldol reaction between cyclohexanone
(10 mmol) and silyl enol ether 7a (10 mmol) catalyzed by 2 (0.5
mmol) in dichloromethane (20 mL) at room temperature.
apparently indicates that 2a, having a TFPB anion, is the most
catalytically active. The reaction catalyzed by 2c was not
completed under the same conditions, indicating the
deactivation of catalytically active species before the reaction
completion. Although the precise mechanism of the present
reaction is unclear, the highest catalytic activity of 2a can be
correlated to the stronger Lewis acidity of 2a (see DFT
Calculations).15
To our surprise, the aldol reaction catalyzed by salt 2b,
having a CB11H12 anion, was not successful under the same
conditions, although the structure of the silaoxazolinium cation
of 2b is almost identical with that of 2a; just after the start of
the reaction catalyzed by 2b, the reaction abruptly stopped.
This result is evidently due to the rapid deactivation of
catalytically active species. No simple explanation for the rapid
deactivation of catalyst 2b can be offered at the present time.
The most acceptable explanation for the deactivation of catalyst
2b is that the decomposition of the CB11H12 anion took place
via hydride abstraction by an electrophilic species in the
reaction solution, leading to the deactivation of catalytically
active species.16
The Mulliken charges of A and B are given in Table 4. As can
be seen from Figure 6, the MOs, which accept the electron pair
of Lewis bases, are LUMO+1 for A and B, whereas the LUMOs
of A and B are π* MOs of [−OCN−] moieties. The charge
distribution in A and B is intriguing (Table 4). The positive
charges of (N-amidomethyl)dimethyl(X)silanes 3−6 have been
considered to be located around oxygen and nitrogen (Scheme
6, C and C′).4a,c,e Examination of Table 4, however, reveals the
striking features of the charge distribution in A and B: (a) the
quaternary iminium nitrogen atoms, which have a formal
positive charge, are negatively charged; (b) despite the formal
positive charge on the oxygen, the oxygen atoms are negatively
charged; (c) appreciable positive charge is located on the
silicon; (d) the silicon of B, which is a model for 2a,b, carries a
larger positive charge than the silicon of A, which is a model for
2c,d.
The results of Mulliken charge analysis are entirely consistent
with the 29Si NMR analysis of a series of silaoxazolinium salts 2
as well as the observed strong Lewis acidity of silaoxazolinium
salt 2a. The formal positive charge on the nitrogen would be
neutralized by electron release from the five neighboring
hydrogen atoms in geminal positions relative to the nitrogen.17
All of the results obtained from the experimental observations
For further investigation as to the influence of the
counteranions on the catalytic activity of the silaoxazolinium
cations, we attempted the aldol reaction between cyclo-
E
dx.doi.org/10.1021/om400017f | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
TFPB anion of 2a opens a coordination site on the silicon,
which permits the strong electrophilic activation of carbonyl
compounds by coordination. In the case of trimethylsilyl-
substituted silaoxazolinium cations 2c,d, two electron-releasing
TMS groups on the central silicon neutralize the positive
charge of the central silicon (0.559) to reduce the Lewis acidity.
Moreover, sterically demanding TMS groups would hinder the
approach of carbonyl compounds to the silicon, thereby
disturbing the activation of carbonyl compounds by coordina-
tion to the silicon.
CONCLUSION
■
DFT calculations of a coordination-free silaoxazolinium cation
predicted the significant Lewis acidity of the silicon atom. In
order to demonstrate the results of the calculation, a series of
silaoxazolinium salts with weakly coordinating anions such as
TFPB− and CB11H12 were synthesized. X-ray crystal analysis
−
of the silaoxazolinium salts has revealed that dimethyl-
substituted silaoxazolinium salts 2a,b are nearly completely
free from the coordination of counteranions, whereas the
silaoxazolinium salts 2b,c, in which the silicon atoms are
substituted with two trimethylsilyl groups, are weakly
coordinated by counteranions.
Silaoxazolinium salt 2a, with a TFPB anion, exhibits effective
catalytic activity for the Mukaiyama aldol reaction of poorly
reactive ketones with low reactive silyl enol ethers, giving the
corresponding aldol products in high yields. The counteranions
as well as the substituents on the silicon exert a strong influence
on the catalytic activities of silaoxazolinium salts.
Figure 6. Molecular orbitals of model compounds A and B calculated
at the B3LYP/6-31G(d) level.
In comparison with widely used strong silicon Lewis acids
such as TMSOTf, TMSNTf2, and TMSI, the Lewis acid
strength of 2a is evidently lower.18 For example, the catalytic
Mukaiyama aldol reaction between Ph(CH2)2CHO and 7a
catalyzed by 2a did not proceed at −10 °C (Table 3, entry 4),
while the same reaction catalyzed by TMSOTf, TMSNTf2, and
TMSI smoothly proceeds at −78 °C.18 However, despite the
higher Lewis acid strength, these conventional silicon Lewis
acids failed to promote the Mukaiyama aldol reaction of
unactivated ketones in spite of the optimization of the reaction
conditions,3b,19 though the same reaction is smoothly catalyzed
by 2a. The discrepancy between the catalytic efficiency and the
Lewis acid strength is highly suggestive of a different reaction
mechanism of the aldol reaction catalyzed by 2a.
Table 4. Mulliken Charges for Model Compounds A and B
at the B3LYP/6-31G(d) Level
A
B
Si(central)
Si(terminal)
Si(terminal)
O
0.559
0.705
Si
1.039
−0.714
−0.715
−0.585
−0.495
0.615
methyl carbon bonded to Si
0.714
methyl carbon bonded to Si
−0.572
−0.485
0.593
O
N
N
amido C
amido C
Scheme 6
As a catalyst, silaoxazolinium salt 2a possesses several
advantages over other metal Lewis acids: (a) 2a is air stable
and can be handled in air, whereas metal Lewis acids are usually
highly sensitive to moisture;18 (b) the aldol reaction catalyzed
by 2a takes place smoothly at room temperature without use of
low-temperature conditions; (c) more importantly, the
silaoxazolinium-catalyzed aldol reaction shows wider substrate
scope, including not only aldehydes but also unactivated
ketones. Further investigations focused on the application of
the silaoxazolinium catalysts to other organic reactions are
underway.
and the DFT calculations, which fully explain the Lewis
acidities of silaoxazolinium cations, indicate the substantial
contribution of resonance structure C″ (Scheme 6), in which
the charge distributions are totally different from the formal
charge distribution.
DFT calculations throw light on the substituent effect on the
Lewis acid strength of the silicon. The highest catalytic activity
for 2a among a series of silaoxazolium salts can be explained by
the largest positive charge on the silicon (1.039), leading to the
strong Lewis acidity. In addition, the weakly coordinating
EXPERIMENTAL SECTION
General Remarks. All manipulations were carried out under a
nitrogen atmosphere using Schlenk tube techniques. H (500 MHz),
■
1
13C (125.4 MHz), and 29Si (99.0 MHz) NMR spectra were recorded
on a JEOL LA-500 spectrometer. Chemical shifts were referenced to
CH2Cl2 (5.31 ppm) for 1H NMR, CD2Cl2 (53.62 ppm) for 13C NMR,
and Me4Si (0 ppm) for 29Si NMR. Infrared spectra were measured on
a JASCO FT/IR-5000 instrument in Nujol mulls prepared under inert
F
dx.doi.org/10.1021/om400017f | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
atmosphere. Solvents were purified as follows: hexane by distillation
from benzophenone ketyl under nitrogen and dichloromethane by
distillation from calcium hydride. All the above solvents were distilled
just prior to use. Silyl enol ethers and silyl ketene acetals were prepared
by literature procedures.20 Sodium tetrakis[3,5-bis(trifluoromethyl)-
phenyl]borate (Na[TFPB]) was obtained from Wako Chemical Co.
Cs[CB11H12] was prepared by the literature procedure.6a Compound
1a was prepared by the literature procedure,7a and compound 1b was
prepared from commercially available chloro(chloromethyl)-
dimethylsilane (Aldrich) and N-methyl-N-(trimethylsilyl)acetamide
(Tokyo kasei) according to the literature procedure.21
2,3-Dimethyl-5,5-bis(trimethylsilyl)-5-silaoxazolinium 1-
Carba-closo-dodecaborate (2d). A 20 mL Schlenk tube was
charged with 1b (0.088 g, 0.300 mmol) and cesium 1-carba-closo-
dodecaborate (0.087 g, 0.315 mmol) under nitrogen. To this mixture
was added anhydrous dichloromethane (2 mL), and the reaction
mixture was further stirred at room temperature for 12 h. The
precipitated cesium chloride was separated by transfer of the
dichloromethane solution into another flask using cannula. The
solution was condensed under reduced pressure to afford a colorless
solid, which was recrystallized from CH2Cl2/hexane at −30 °C to give
2d as colorless crystals (0.113 g, 0.280 mmol, 94%). Mp: 132−134 °C.
1
IR (Nujol): 2518, 1625, 1455, 1247, 1018, 844, 757, 501 cm−1. H
2,3,5,5-Tetramethyl-5-silaoxazolinium Tetrakis[3,5-bis-
(trifluoromethyl)phenyl]borate (2a). A 20 mL Schlenk tube was
charged with 1a (0.054 g, 0.300 mmol) and sodium tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate (Na[TFPB]; 0.290 g, 0.315 mmol)
under nitrogen. To this mixture was added anhydrous dichloro-
methane (2 mL), and the reaction mixture was further stirred at room
temperature for 2 h. The precipitated sodium chloride was separated
by transfer of the dichloromethane solution into another flask using
cannula. The solution was condensed under reduced pressure to afford
a colorless solid, which was recrystallized from CH2Cl2/pentane at
−30 °C to give 2a as colorless crystals (0.290 g, 0.287 mmol, 96%).
Mp: 110−113 °C. IR (Nujol): 2750, 1635, 1355, 1276, 1116, 887, 860,
NMR (CD2Cl2): δ 0.31 (s, 18 H), 1.04−2.35 (m, 12 H), 2.44 (s, 3 H),
3.46 (s, 3 H), 3.55 (s, 2 H). 13C NMR-DEPT (CD2Cl2): δ −2.0
(CH3), 17.6 (CH3), 39.8 (CH3), 41.8 (CH2), 52.1 (carborane-C),
178.1 (CO). 29Si NMR (CD2Cl2): δ −13.2 (terminal Si), 27.3 (central
Si). Anal. Calcd for C11H38B11NOSi3: C, 32.74; H, 9.49; N, 3.47.
Found: C, 32.98; H, 9.30; N, 3.45.
General Procedure for the Catalytic Aldol Addition. To a
solution of 2a (0.030 g, 0.030 mmol) in anhydrous dichloromethane
(2 mL) were added silyl enol ether or silyl ketene acetal (0.66−1.2
mmol) and ketone or aldehyde (0.60 mmol) dropwise at room
temperature. After stirring for an additional 10 h at room temperature,
the reaction mixture was poured into a saturated NH4Cl solution (6
mL) and stirred for a few minutes and then extracted with ether (20
mL × 2). The extract was dried over anhydrous Na2SO4 and
concentrated under reduced pressure. The residue was purified by
preparative thin -layer chromatography using hexane/ethyl acetate (9/
1) as eluent to furnish an aldol adduct in the yields shown in Table 3.
The isomeric ratio was determined by 500 MHz 1H NMR
spectroscopy.
1
809, 779, 711, 682, 671 cm−1. H NMR (CD2Cl2): δ 0.63 (s, 6 H),
2.34 (s, 3 H), 3.12 (s, 2 H), 3.30 (s, 3 H), 7.67 (s, 4 H), 7.83 (s, 8 H).
13C NMR-DEPT (CD2Cl2): δ −1.5 (CH3), 16.9 (CH3), 39.6 (CH2),
42.9 (CH3), 118.0 (TFPB-C4), 125.1 (q, 1JFC = 271.7 Hz, CF3), 129.4
(q, 2JFC = 31.0 Hz, TFPB-C3,5), 135.3 (TFPB-C2,6), 162.1 (q, 1JBC
=
51.2 Hz, B-C1), 178.7 (CO). 29Si NMR (CD2Cl2): δ 46.1. Anal. Calcd
for C38H26BF24NOSi: C, 45.30; H, 2.60; N, 1.39. Found: C, 45.06; H,
2.46; N, 1.20.
All compounds in Table 3 have been previously isolated and
characterized. Their Chemical Abstracts registry numbers are as follows:
8, 6397-70-2; 9, 57213-26-0; 10, 60669-64-9; syn-11, 103108-36-7;
anti-11, 103108-37-8; 12, 33026-24-3.
2,3,5,5-Tetramethyl-5-silaoxazolinium 1-Carba-closo-dodec-
aborate (2b). A 20 mL Schlenk tube was charged with 1a (0.054 g,
0.300 mmol) and cesium 1-carba-closo-dodecaborate (0.087 g, 0.315
mmol) under nitrogen. To this mixture was added anhydrous
dichloromethane (2 mL), and the reaction mixture was further stirred
at room temperature for 12 h. The precipitated cesium chloride was
separated by transfer of the dichloromethane solution into another
flask using cannula. The solution was condensed under reduced
pressure to afford a colorless solid, which was recrystallized from
CH2Cl2/pentane at room temperature to give 2b as colorless crystals
(0.081 g, 0.281 mmol, 93%). Mp: 112−114 °C. IR (Nujol): 2601,
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving crystallographic data for 2a−d, figures giving
13C NMR and 1H NMR spectra of new compounds, and a table
giving crystallographic data and experimental parameters for the
structure analysis of 2a−d. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
1637, 1540, 1305, 1153, 1022, 850, 771, 721, 592 cm−1. H NMR
(CD2Cl2): δ 0.66 (s, 6 H), 1.00−2.34 (m, 12 H), 2.45 (s, 3 H), 3.24 (s,
2 H), 3.41 (s, 3 H). 13C NMR-DEPT (CD2Cl2): δ −0.4 (CH3), 17.9
(CH3), 40.2 (CH3), 43.3 (CH2), 52.1 (carborane-C), 177.7 (CO). 29Si
NMR (CD2Cl2): δ 49.6. Anal. Calcd for C7H26B11NOSi: C, 29.26; H,
9.12; N, 4.87. Found: C, 29.05; H, 8.88; N, 4.66.
AUTHOR INFORMATION
Corresponding Author
*Y.H.: e-mail, hatanaka@a-chem.eng.osaka-cu.ac.jp; fax, +81
6605-6-2978; tel, +81 6605-6-2979.
■
2,3-Dimethyl-5,5-bis(trimethylsilyl)-5-silaoxazolinium
Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (2c). A 20 mL
Schlenk tube was charged with 1b (0.088 g, 0.300 mmol) and sodium
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na[TFPB]; 0.290 g,
0.315 mmol) under nitrogen. To this mixture was added anhydrous
dichloromethane (2 mL), and the reaction mixture was further stirred
at room temperature for 2 h. The precipitated sodium chloride was
separated by transfer of the dichloromethane solution into another
flask using cannula. The solution was condensed under reduced
pressure to afford a colorless solid, which was recrystallized from
CH2Cl2/hexane at −30 °C to give 2c as colorless crystals (0.327 g,
0.291 mmol, 97%). Mp: 103−107 °C. IR (Nujol): 2724, 2362, 1631,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by a Grant-in-Aid for Science
Research (C) (20550101) from JSPS.
■
REFERENCES
■
(1) (a) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J.
Am. Chem. Soc. 1999, 121, 5001−5008. (b) Sekiguchi, A.; Matsuno, T.;
Ichinohe, M. J. Am. Chem. Soc. 2000, 122, 11250−11251. (c) Lambert,
J. B.; Zhao, Y.; Zhang, S. M. J. Phys. Org. Chem. 2001, 14, 370−379.
(d) Kim, K.-C.; Reed, C. A.; Elliot, D. W.; Mueller, L. J.; Tham, F.;
Lin, L.; Lambert, J. B. Science 2002, 297, 825−827. (e) Matthew, N.;
Reed, C. A. Organometallics 2011, 30, 4798−4800. (f) Romanato, P.;
Duttwyler, S.; Linden, A.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem.
Soc. 2010, 132, 7828−7829. (g) Romanato, P.; Duttwyler, S.; Linden,
A.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 2011, 133, 11844−
11846. (h) Khaliman, A. Y.; Lin, Z. H.; Simionescu, R.; Vyboishchicov,
1
1276, 968, 935, 887, 775, 669, 603, 568 cm−1. H NMR (CD2Cl2): δ
0.25 (s, 18 H), 2.33 (s, 3 H), 3.32 (s, 3 H), 3.38 (s, 2 H), 7.57 (s, 4 H),
7.72 (s, 8 H). 13C NMR-DEPT (CD2Cl2): δ −2.5 (CH3), 17.3 (CH3),
39.6 (CH3), 41.3 (CH2), 117.9 (TFPB-C4), 125.0 (q, 1JFC = 272.5 Hz,
2
CF3), 129.5 (q, JFC = 31.7 Hz, TFPB-C3,5), 135.2 (TFPB-C2,6),
162.15 (q, 1JBC = 49.8 Hz, B-C1), 178.5 (CO). 29Si NMR (CD2Cl2): δ
−12.3 (terminal Si), 31.3 (central Si). Anal. Calcd for
C42H38BF24NOSi3: C, 44.89; H, 3.41; N, 1.24. Found: C, 44.67; H,
3.25; N, 1.39.
G
dx.doi.org/10.1021/om400017f | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
S. F.; Nikonov, G. I. Angew. Chem., Int. Ed. 2007, 46, 4530−4533.
(i) Inoue, S.; Ichinohe, M.; Yamaguchi, T.; Sekiguchi, A. Organo-
metallics 2008, 27, 6053−6058. (j) Kessler, M.; Knapp, C.; Sagawa, V.;
Organometallic and Polymer Chemistry; Wiley: Chichester, U.K., 2000;
pp 13−26.
(12) (a) Kost, D.; Kalikhman, I. In The Chemistry of Organic Silicon
Compounds; Apeloig, Y., Rappoport, Z., Eds.; Wiley: Chichester, U.K.,
1998; Vol. 2, pp 1339−1445. (b) Gostevskii, B.; Silbert, G.; Deuerlein,
G.; Kocher, N.; Voronkov, M. G.; Kalikhman, I.; Kost, D.
Organometallics 2005, 24, 2913−2920. (c) Kost, D.; Kingston, V.;
Gostevskii, B.; Ellern, A.; Stalke, D.; Walfort, B.; Kalikhman, I.
Organometallics 2002, 21, 2293−2305. (d) Gostevskii, B.; Zamstein,
N.; Korlyukov, A. A.; Stalke, D.; Baukov, Y.; Botoshansky, M.; Kaftory,
N.; Kocher, N.; Kalikhman, I.; Kost, D. Organometallics 2006, 25,
5416−5423. (e) Bassindale, A.; Parker, D. J.; Taylor, P. G.; Turtle, R.
Z. Anorg. Allg. Chem. 2009, 635, 1288−1294. (f) Denmark, S. E.;
Eklov, B. M.; Yao, P. J.; Eastgate, M. D. J. Am. Chem. Soc. 2009, 131,
11770−11787. (g) Bockholt, A.; Jutzi, P.; Mix, A.; Neumann, B.;
Stammler, A.; Stammler, H.-G. Z. Anorg. Allg. Chem. 2009, 635, 1326−
1334. (h) Kalikhman, I.; Gostevskii, B.; Girshberg, O.; Kocher, N.;
Kost, D. J. Organomet. Chem. 2003, 686, 202−214. (i) Kalikhman, I.;
Krivonov, S.; Lameyer, L.; Stalke, D.; Kost, D. Organometallics 2001,
20, 1053. (j) Ovchinikov, Y. E.; Pogozhikh, S. A.; Razumovskaya, A.
G.; Shopov, A. G.; Kramarova, E. P.; Bylikin, S. Y.; Negrevetsky, V. V.;
Baukov, Y. I. Russ. Chem. Bull. 1998, 47, 967−978. (k) Chauhan, M.;
Chuit, C.; Corriu, R. J. P.; Reye, C. Tetrahedron Lett. 1996, 37, 845−
848.
Scherer, H.; Uzun, R. Inorg. Chem. 2010, 49, 5223−5230. (k) Muther,
̈
K.; Frorich, R.; Murk-Lichtenfeld, C.; Grimme, S.; Oesterreich, M. J.
̈
̈
Am. Chem. Soc. 2011, 133, 12422−12444. For reviews, see:
(l) Lambert, J. B.; Kania, L.; Zhang, S. Chem. Rev. 1995, 95, 1191−
1201. (m) Reed, C. A. Acc. Chem. Res. 1998, 31, 325−332. (n) Mueller,
T. Adv. Organomet. Chem. 2005, 53, 155−215. (o) Maerker, C.;
Shleyer, P. v. R. In The Chemistry of Organic Silicon Compounds;
Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2,
pp 513−555. (p) Lickiss, P. D. In The Chemistry of Organic Silicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K.,
1998; Vol. 2, pp 557−594. (q) Lee, V.; Sekiguchi, A. In Reviews of
Reactive Intermediate Chemistry; Platz. M. S., Moss. R. A., Jones, M., Jr.,
Eds.; Wiley: New York, 2007; pp 47−120.
(2) (a) Avelar, A.; Tham, F. S.; Reed, C. A. Angew. Chem., Int. Ed.
2009, 48, 3491−3493. (b) Duttwyler, S.; Douvris, C.; Fackler, N. L. P.;
Tawm, F. S.; Reed, C. A.; Baldridge, K. K.; Siegel, J. S. Angew. Chem.,
Int. Ed. 2010, 49, 7519−7522. (c) Allemann, O.; Duttwyler, S.;
Romanato, P.; Baldridge, K. K.; Siegel, J. S. Science 2011, 332, 573−
577. (d) Schafer, A.; Saak, W.; Detlev, H.; Muller, T. Angew. Chem., Int.
Ed. 2012, 51, 2981−2984. (e) Reed, C. A. Acc. Chem. Res. 2010, 43,
121−128 and references therein.
(3) (a) Johannsen, M.; Jørgensen, K. A.; Helmchen, G. J. Am. Chem.
Soc. 1998, 120, 7637−7638. (b) Hara, K.; Akiyama, R.; Sawamura, M.
Org. Lett. 2005, 7, 5621−5623. (c) Klare, H. F. T.; Bergander, K.;
Oestreich, M. Angew. Chem., Int. Ed. 2009, 48, 9077−9079. (d) Klare,
H. F. T.; Oesterreich, M. Dalton Trans. 2010, 39, 9176−9184.
(e) Muhammad, F. I.; Langer, P.; Schultz, A.; Villinger, A. J. Am. Chem.
Soc. 2011, 133, 21016−21027 and references therein.
(13) (a) Oishi, M.; Aratake, S.; Yamamoto, H. J. Am. Chem. Soc.
1998, 120, 8271−8272. For a review, see: (b) Gennari, C. In
Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, U.K., 1991; Vol. 2, pp 629−660. For
enantioselective Mukaiyama aldol reactions of unactivated ketones
with ketene silyl acetals see: (c) Adachi, S.; Harada, T. Eur. J. Org.
Chem. 2009, 3661−3671.
(14) (a) Hollis, T. K.; Robinson, N. P.; Bosnich, B. Tetraheron Lett.
1992, 33, 6423−6426. (b) Ohki, H.; Wada, M.; Akiba, K. Tetrahedron
Lett. 1988, 29, 6423−6426. (c) Marx, A.; Yamamoto, H. Angew. Chem.,
Int. Ed. 2000, 39, 178.
(4) (a) Macharashvili, A. A.; Shklover, V. E.; Struchkov, Y. T.;
Olenova, G. I.; Kramanova, E. P.; Shipov, A. G.; Baukov, Y. I. J. Chem.
Soc., Chem. Commun. 1988, 683−685. (b) Bassindale, A. R.;
Borbaruah, M. J. Chem. Soc., Chem. Commun. 1991, 1499−1501.
(c) Bassindale, A. R.; Borbaruah, M.; Simon, J. G.; David, J. P.; Tayler,
P. G. J. Chem. Soc., Perkin Trans. 2 1999, 2099−2109. (d) Bassindale,
A. R.; Parker, D. J.; Tayler, P. G.; Auner, N.; Herrscaft, B. J. Organomet.
Chem. 2003, 667, 66−72. (e) Muhammad, S.; Bassindale, A. R.;
Taylor, P. G.; Male, L.; Coles, S. J.; Hursthouse, M. B. Organometallics
2011, 30, 564−571 and references therein. For reviews, see:
(f) Bassindale, A. R.; Glynn, S. J.; Taylor, P. G. In The Chemistry of
Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley:
Chichester, U.K., 1998; Vol. 2, pp 495−511.
(15) For the mechanism of the Mukaiyama aldol reaction, see:
(a) Cheon, C. H.; Yamamoto, H. Org. Lett. 2010, 12, 2476−2479.
(b) Hiraiwa, Y.; Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 2006,
1837−1844. (c) Wong, C. T.; Wong, M. W. J. Org. Chem. 2007, 72,
1425−1430. (d) Wang, L.; Wah, W. H. Tetrahedron Lett. 2008, 49,
3916−3920. (e) Wong, C. T.; Wong, M. W. J. Org. Chem. 2005, 70,
124−131. (f) Ishihara, K.; Hiraiwa, Y.; Yamamoto, H. Chem. Commun.
2002, 1564−1565.
(16) Strauss, S. H. Chem. Rev. 1993, 93, 927−942.
(17) Gostevskii, B.; Zamstein, N.; Korlyukov, A. A.; Baukov, Y. I.;
Botoshansky, M.; Kaftory, M.; Kocher, N.; Stalke, D.; Kalikhman, I.;
Kost, D. Organometallics 2006, 25, 5416−5423.
(5) (a) Olsson, L.; Ottoson, C.-H.; Cremer, D. J. Am. Chem. Soc.
1995, 117, 7460−7479. (b) Xie, Z.; Benesi, R.; Reed, C. A.
(18) For reviews, see: (a) Dilman, A. D.; Ioffe, S. L. Chem. Rev. 2003,
103, 733−772. (b) Ishihara, K.; Yamamoto, H. In Modern Aldol
Reactions; Mahrwalt, R., Ed.; Wiley-VCH: Weinheim, Germany, 2004;
Vol. 2, pp 25−68. (c) Hosomi, A.; Miura, K. In Acid Catalysis in
Modern Organic Synthesis; Yamamoto, H., Ishihara, K., Eds.; Wiley-
VCH: Weinheim, Germany, 2008; Vol. 1, pp 469−516.
(19) Mukai, C.; Hashizume, S.; Nagami, K.; Hanaoka, M. Chem.
Pharm. Bull. 1990, 38, 1509−1512.
(20) (a) House, H. O.; Czuba, L. J.; Gall, M.; Olmstead, H. D. J. Org.
́
Organometallics 1995, 14, 3933−3041. (c) Ottoson, C.-H.; Szabo, K.
J.; Cremer, D. Organometallics 1997, 16, 2377−2385. (d) Ottoson, C.-
H.; Cremer, D. Organometallics 1996, 15, 5309−5320.
(6) (a) Shelly, K.; Finster, D. C.; Lee, Y. J.; Scheidt, W. R.; Reed, C.
A. J. Am. Chem. Soc. 1985, 107, 5955−5959. For a review of weakly
coordinating anions, see: (b) Reed, C. A. Chem. Rev. 1993, 93, 927−
942. (c) Krossing, I.; Raabe, I. Angew. Chem., Int. Ed. 2004, 43, 2066−
2090. (d) Reed, C. A. Acc. Chem. Res. 2010, 43, 121−128.
(7) (a) El-Sayed, I.; Hatanaka, Y.; Muguruma, C.; Shimada, S.;
Tanaka, M.; Koga, N.; Mikami, M. J. Am. Chem. Soc. 1999, 121, 5095−
5096. (b) El-Sayed, I.; Hatanaka, Y.; Onozawa, S.-y.; Tanaka, M. J. Am.
Chem. Soc. 2001, 123, 3597−3598.
́
Chem. 1969, 34, 2324−2336. (b) Eusebio, J.; Cruz-Sanchez, Y. J. Org.
Chem. 1988, 53, 3334−3338.
(21) Hillyard, R. W., Jr.; Ryan, C. M.; Yoder, C. H. J. Organomet.
Chem. 1978, 153, 369−377.
(8) Kertsnus-Banchik, E.; Gostevskii, B.; Botoshansky, M.;
Kalikhman, I.; Kost, D. Organometallics 2010, 29, 5435−5445.
(9) Mantina, M.; Chamberlin, A. C.; Valero, C. J.; Truhar, D. G. J.
Phys. Chem. 2009, 113, 5806−5812.
(10) Williams, E. A. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.: Wiley: Chichester, U.K., 1989; Vol. 1, pp
511−554.
(11) (a) Marsmann, H. In NMR 17, Basic Principles and Progress:
Oxygen-17 and Silicon 29; Diel, P., Fluck, E., Kosfeld, R., Eds.;
Springer: Berlin, 1981; pp 65−235. (b) Brook, M. A. Silicon in
H
dx.doi.org/10.1021/om400017f | Organometallics XXXX, XXX, XXX−XXX