Stereocontrolled synthesis of tertiary silanes via optically pure 1,3,2-oxazasilolidine derivatives

Article abstract of DOI:10.1016/j.jorganchem.2009.02.022

Optically pure 1,3,2-oxazasilolidine derivatives were synthesized from a chiral 1,2-amino alcohol. These heterocyclic compounds containing a stereogenic silicon atom produced tertiary silanes with excellent optical purity through successive reactions with

Full text of DOI:10.1016/j.jorganchem.2009.02.022

Journal of Organometallic Chemistry 694 (2009) 2171–2178
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Stereocontrolled synthesis of tertiary silanes via optically
pure 1,3,2-oxazasilolidine derivatives
*
Natsuhisa Oka, Masahiko Nakamura, Naomi Soeda, Takeshi Wada
Department of Medical Genome Sciences, Graduate School of Frontier Sciences, The University of Tokyo, Bioscience Building 702, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8562, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Optically pure 1,3,2-oxazasilolidine derivatives were synthesized from a chiral 1,2-amino alcohol. These
heterocyclic compounds containing a stereogenic silicon atom produced tertiary silanes with excellent
optical purity through successive reactions with Grignard reagents and diisobutylaluminum hydride. Ste-
reochemical course of the reactions of the oxazasilolidine at the chiral silicon atom was elucidated based
on the absolute configurations of the products and the substrate which were determined by chiral HPLC
and X-ray crystallographic analyses.
Received 26 December 2008
Received in revised form 17 February 2009
Accepted 20 February 2009
Available online 5 March 2009
Keywords:
Stereocontrolled synthesis
Silane
Ó 2009 Elsevier B.V. All rights reserved.
Organosilicon compound
Oxazasilolidine
Amino alcohol
1. Introduction
thesize optically active Si-stereogenic organosilicon compounds in
a stereocontrolled manner. In this paper, we wish to describe the
Optically active organosilicon compounds containing a stereo-
genic silicon atom have been studied for a wide range of applica-
tions, such as chiral auxiliaries [1], reagents [2], and resolving
agents [3], drug candidates [4], stereoregulated polymers and their
monomers [5]. Since these compounds retain unique properties
that do not exist in the carbon-based counterparts, they have
applications that are clearly differentiated from those of the car-
bon-based molecules (e.g., chiral auxiliaries that can be used as
C–H or C–OH equivalents via substitution with Fꢀ [1d] or the Ta-
mao oxidation [1a], reagents for asymmetric hydrosilylations
[2a,b,d,f], optically active polymers that retain flexibility, thermal
stability, conductivity, nonlinear optical properties, etc. [5]).
However, the preparation of optically active organosilicon com-
pounds is still a challenging task and mostly relies on the classical
optical resolution of a single starting material, 1-NpPhMeSiOMen
[1-Np = 1-naphthyl, Men = (–)-menthyl], which was developed by
Sommer et al. [6] due to the lack of an efficient, versatile method
to synthesize these compounds in a stereocontrolled manner.
Although not a few methods have been developed to synthesize
these compounds by using optical resolutions [6,7] or stereocon-
trolled reactions [8] and some of them provide the products with
high optical purity [6,7,8a–g], none of these methods can provide
a wide range of Si-stereogenic organosilicon compounds. With this
background, we sought to develop a new versatile approach to syn-
development of a versatile method that uses optically pure 1,3,2-
oxazasilolidine derivatives as key intermediates to synthesize ter-
tiary silanes with excellent optical purity.
2. Results and discussion
2.1. Synthetic strategy
The strategy of the present method is outlined in Scheme 1.
Enantiopure 1,2-amino alcohols 1 are used as chiral sources and al-
lowed to react with a prochiral silane derivative 2, which has two
leaving groups on its silicon atom, to synthesize 1,3,2-oxazasiloli-
dine derivatives 3. It is expected that the 1,3,2-oxazasilolidine
key intermediates are formed with high stereoselectivity by an
appropriate design of the 1,2-amino alcohol. Since Si–N bonds
are more reactive toward nucleophiles than Si–O bonds in general
[8f,g,9], an equimolar amount of a C-nucleophile would exclusively
substitute the amino group on the chiral silicon atom of 3 to afford
silyl ethers 4. The silyl ethers 4 are expected to be useful as precur-
sors of tertiary and quaternary silanes, or silyl ethers via nucleo-
philic attack of hydride, C-nucleophiles, or O-nucleophiles,
respectively. In this study, we aimed to synthesize optically active
tertiary silanes, which have much wider applications (e.g., mono-
mers of stereoregular silicon-containing polymers [5a,b], reagents
for asymmetric hydrosilylations [2a,b,d,f], and precursors of a vari-
ety of optically active organosilicon compounds via Si–H modifica-
tions [10]) than the other two potential targets. Phosphorus-based
* Corresponding author. Tel./fax: +81 4 7136 3612.
E-mail address: wada@k.u-tokyo.ac.jp (T. Wada).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.02.022

2172
N. Oka et al. / Journal of Organometallic Chemistry 694 (2009) 2171–2178
Table 1
R1 R2
HO
R5
HN R3
R4
Synthesis of 1,3,2-oxazasilolidines 3a–g.
R1
X
R2
X
Si
R3
+
Si
O
N
stereoselective
Ph
O
Me
N
HO
R5
HN R3
R4
R4
PhMeSi(NMeCOCF3)2 (2a, 1.1 equiv)
R5
Si
1
2
R3
3
THF, rt, overnight
R6
M
stereospecific
R4
1a
−
g
R5
3a−g
H−
stereospecific
R2
R1
R6
O
R2
R1
R6
H
R4
∗
∗
a
Entry
1
R3
R4
R5
yield
RSi:SSi
Si
Si
R3
1
2
3
4
5
6
7
8b
a
b
c
d
e
e
f
Ph
Ph
Ph
Ph
H
i-Pr
H
Ph
Ph
Ph
Ph
t-Bu
i-Pr
H
Ph
H
H
H
66%
66%
67%
90%
90%
64%
–
62:38
78:22
80:20
85:15
89:11
>99:1c
–
N
H
4
R5
5
Scheme 1. Stereocontrolled synthesis of Si-stereogenic tertiary silanes 5 via 1,3,2-
oxazasilolidine intermediates 3.
p-And
p-And
p-NO2C6H4
Ph
H
H
g
83%
80:20
five-membered rings derived from chiral 1,2-amino alcohols
(1,3,2-oxazaphospholidines) have been successfully used to syn-
thesize optically pure organophosphorus compounds, such as li-
gands and biologically active compounds [11]. We have also
developed a method to synthesize stereoregulated oligonucleotide
analogues with P-modifications via 1,3,2-oxazaphospholidine
intermediates [12]. These studies have also inspired the strategy
of this study.
It has been reported that optically pure 1,3,2-oxazasilolidine
derivatives (Fig. 1, 6) were synthesized by using the corresponding
amino acids, and used as precursors of optically active Si-stereo-
genic organosilicon compounds [8f,g]. However, a partial loss of
optical purity occurred due to the epimerization of reaction inter-
mediates. Silicon-containing heterocycles derived from C2-sym-
metric diols 7 have also been developed and the successive
nucleophilic substitutions at their silicon atoms afforded optically
active silanes [8d,e]. However, stereoselectivity of the reactions
was strongly dependent on the structure of nucleophiles because
the stereoselectivity is controlled only by the stereoelectronic ef-
fects of 7 and the leaving ability of the two hydroxy groups at
the silicon atoms is virtually the same. In contrast, a stoichiometric
nucleophile is expected to substitute the amino group of the 1,3,2-
oxazasilolidine 3 exclusively due to the significant difference in
reactivity between the Si–N and Si–O bonds.
a
Determined by 1H NMR.
Racemic 1g was used.
Purified by precipitation from Et2O.
An = anisyl.
b
c
d
atives 3a–g were extremely sensitive to moisture, crude 3a–g were
obtained upon removal of any volatile reagents under inert atmo-
sphere, and analyzed by 1H NMR to determine the diastereoselec-
tivity of the reactions. The analysis showed that the desired
oxazasilolidine derivatives 3a–g were generated with modest to
good diastereoselectivity (d.r. = 62:38–89:11) (Table 1). Oxazasilo-
lidine derivatives having a substituent at the four-position were
prone to give higher diastereoselectivity than those having a sub-
stituent at the five-position (entry 1 vs. 2, 3 vs. 4). Formation of
an oxazasilolidine was not observed when a p-nitrophenyl group
was used as the N-substituent of 1,2-amino alcohol (entry 7). To
our surprise, when crude 3e (d.r. = 89:11, entry 5) was washed
with dry Et2O, only the major diastereomer of 3e remained as crys-
tals and isolated in 64% yield (entry 6). An X-ray crystallographic
analysis of the resultant 3e showed that the absolute configuration
of the chiral silicon atom was RSi (Fig. 2). It should be noted that 3e
as well as the other oxazasilolidines were configurationally stable
at r.t. but underwent epimerization at an elevated temperature. For
example, d.r. of crude 3e was lowered upon heating at 90 °C for 7 h
in dry toluene (RSi:SSi = 60:40).
2.2. Synthesis of optically active 1,3,2-oxazasilolidine derivatives
2.3. Nucleophilic substitution of 1,3,2-oxazasilolidine derivatives with
Grignard reagents
Firstly, the stereoselective synthesis of 1,3,2-oxazasilolidine
derivatives was investigated. A variety of chiral 1,2-amino alcohols
(Table 1, 1a–g) were allowed to react with bis(N-methyl-2,2,2-tri-
fluoroacetamido)methylphenylsilane 2a in THF under anhydrous
conditions. The prochiral silane 2a was employed because it does
not generate HCl upon reaction with amino alcohols and the result-
ing N-methyl-2,2,2-trifluoroacetamide can be removed in vacuo
more easily than N-methylacetamide, which would be generated
from bis(N-methylacetamido)methylphenylsilane, the starting
material of 6 [8g,h]. Since the resultant 1,3,2-oxazasilolidine deriv-
Next, the diastereopure 1,3,2-oxazasilolidine 3e thus obtained
was allowed to react with a variety of Grignard reagents. Subse-
quent treatment of the products with Boc2O and aqueous work-
up afforded the desired silyl ethers 4a–g with excellent diastereo-
purity (d.r. = 94:6–99:1, summarized in Table 2) in modest to good
R1 R2
Si
R1 R2
Si
R1 R2
Si
R3
R3
O
O
O
N
O
N
R4
R4
R5
O
MeO
OMe
3
6
7
Fig. 1. Silicon-containing heterocyclic intermediates for asymmetric synthesis of
silanes.
Fig. 2. X-ray crystal structure of (2RSi,4S)-3e.

N. Oka et al. / Journal of Organometallic Chemistry 694 (2009) 2171–2178
2173
Table 2
Reaction of (2RSi,4S)-3ea with Grignard reagents.
Ph
Me
R6
O
Me
Ph
∗
Si
1) R6MgBr (1.1 equiv), −78 °C
Si
Boc
N
An-p
O
N
2) Boc2O, rt, overnight
An-p
Ph
(2RSi,4S)-3e
Ph
4a−g
Entry
4
R6
solvent
Timeb (h)
Yieldc (%)
d.r.d
1
2e
3
4
5
6
7
8
9
a
a
b
c
c
d
e
f
1-Np
1-Np
THF
THF
THF
THF
2
2
6
2
6
2
2
2
2
55
60
54
57
53
44
21
59
43
99:1
1:99
99:1
95:5
99:1
96:4
94:6
96:4
95:5
o-MeOC6H4
o-MeC6H4
o-MeC6H4
CH2 = CH
CH2 = CHCH2
Me2C = CH
mesityl
THF–toluene (1:4, v/v)
THF
THF
THF
THF
g
a
Diastereopure 3e (d.r. > 99:1) was used.
Reaction time for step 1.
b
c
Isolated yield of 4a–g from 1,2-amino alcohols 1e (3 steps).
d
e
Determined by 1H NMR.
(R)-2-(p-Methoxyanilino)-2-phenylethanol [(R)-1e] was used as the starting material.
total yields from the 1,2-amino alcohol (S)-1e (3 steps) [13,14]. As
we expected, the Grignard reagents substituted the amino group
on the silicon atom virtually exclusively. Each enantiomer of the
1,2-amino alcohol 1e [(R)- and (S)-1e] afforded the corresponding
diastereopure silyl ether (entries 1 and 2), though the absolute
configurations of the silicon atoms could not be assigned at this
point. The diastereopurity of the product was slightly improved
when the reaction was conducted in a less-polar reaction medium
(entries 4 and 5).
[7a,15]. It indicates that the absolute configuration of the silicon
atom of 4a, the precursor of (RSi)-5a is also (RSi) and the reaction
of the 1,3,2-oxazasilolidine (2RSi,4S)-3e with Grignard reagents pro-
ceeded with inversion of configuration at the silicon atom.
2.5. Mechanism of stereoselective formation of 1,3,2-oxazasilolidine
derivatives
In order to elucidate the mechanism of the stereoselective for-
mation of 1,3,2-oxazasilolidine derivatives, we carried out several
experiments described below. Firstly, the reactivity of 2a toward
the amino and the hydroxy groups of the 1,2-amino alcohols used
in this study was investigated to determine whether the amino or
the hydroxy group reacted with the silicon atom of 2a first. Benzyl-
phenylamine and methanol were used as model compounds of the
amino and the hydroxy moieties of the 1,2-amino alcohols, respec-
tively. Thus, 2a was allowed to react with benzylphenylamine
(2 equiv.) in CDCl3, and the reaction was monitored by 1H NMR
to find that no nucleophilic substitution at the silicon atom oc-
curred (Scheme 2). In contrast, addition of MeOH (2 equiv.) to
2.4. Synthesis of optically active Si-stereogenic tertiary silanes by
reduction of silyl ethers
Next, we examined the synthesis of tertiary silanes by reduction
of the diastereopure silyl ethers thus obtained. Lithium aluminum
hydride (LAH) [6] and diisobutylaluminum hydride (DIBAL)
[7a,15], both of which are known to reduce silyl ethers to tertiary
silanes with retention of configuration, were employed. Diastereo-
pure silyl ether 4a was reduced under several different conditions
(Table 3). The results showed that DIBAL gave the desired tertiary
silane 5a virtually stereospecifically, though an extended reaction
at an elevated temperature was required (entry 4). Sterically less-
hindered LAH gave the product with lower optical purity (entries
1 and 2). Chiral HPLC analysis (Daicel CHIRALCEL OD-H) [16] of
the resultant 5a showed that the absolute configuration of the sili-
con atom was RSi [17]. Under these conditions, (RSi)- and (SSi)-
methyl-1-naphthylphenylsilane [(RSi)- and (SSi)-5a], o-anisylmeth-
ylphenylsilane 5b, methylphenyl(o-tolyl)silane 5c were synthe-
sized from the corresponding silyl ethers 4a–c with excellent
optical purity (Table 4). It should be noted that the relatively low-
boiling tertiary silanes obtained from the silyl ethers 4d–f could
not be isolated from the solvent. The silyl ether 4g did not give
the corresponding tertiary silane under the same conditions proba-
bly due to the steric hindrance of the mesityl group on the silicon
atom. Both enantiomers of 5a were obtained with similar yields
and optical purity. The major enantiomer of the resultant 5b was
eluted faster than the minor enantiomer under the same chiral
HPLC analytical conditions, thus tentatively assigned as (RSi)-iso-
mer. Every attempt to separate the enantiomers of 5c by using chi-
ral HPLC and GC resulted in failure. As described above, reductions
of silyl ethers with DIBAL proceed with retention of configuration
Table 3
Reduction of 4a.
reducing agent
OMe
Me
Ph
∗
Me
Ph
∗
Si
Si
O
Boc
N
H
5a
Ph
4a
a
Entry
Reaction conditions
Yield (%)
RSi:SSi
1
2
3
4
LAH (5 equiv.), THF, reflux, 1.5 h
LAH (3 equiv.), Bu2O, 140 °C, 8 h
DIBAL (10 equiv.), hexane, reflux, 3 d
DIBAL (6 equiv.), Bu2O, 140 °C, 2 d
50
27
9
82:18
85:15
93:7
37
98:2
a
Determined by HPLC (Daicel CHIRALCEL OD-H, 0.46 ꢁ 25 cm, hexane, 0.4 mL/
min).

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N. Oka et al. / Journal of Organometallic Chemistry 694 (2009) 2171–2178
Table 4
Me
NMeCOCF3
PhMeSi(NMeCOCF3)2
Synthesis of tertiary silanes 5a–c.
Si
HO HN An-p
(2a, 1.1 equiv)
Ph
O
HN An-p
Me
Ph
R6
O
THF-d8, rt
∗
Ph
R6
H
DIBAL
Me
Ph
Si
∗
1e
Ph
Si
Boc
N
CF3CONHMe
CF3CONHMe
8
0.2 M
Bu2O, 140 °C, 2 d
5a−c
4a−c
Ph
OMe
a
Ph
O
Me
Entry
4
R6
Yield (%)
RSi: SSi
Si
1
a
a
b
c
1-Np
1-Np
o-MeOC6H4
o-MeC6H4
37
41
68
54
98:2
3:97
97:3
An-p
N
2b
3
4
Not determined
Ph
3e
a
Determined by HPLC (Daicel CHIRALCEL OD-H, 0.46 ꢁ 25 cm, hexane, 0.4 mL/
min).
Scheme 3. Formation of oxazasilolidine 3e and generation of N-methyl-2,2,2-
trifluoroacetamide via successive nucleophilic attacks of hydroxy and amino groups
of 1e.
b
Silyl ether 4a obtained from (R)-1e was used.
the mixture resulted in a quantitative formation of PhMeSi(OMe)2
within 5 min. These experiments indicate that the hydroxy group
of the 1,2-amino alcohol 1a–g reacts with the silicon atom of 2a
first, and the following intramolecular nucleophilic attack of the
amino group generates the oxazasilolidine derivative 3a–g. To
examine the relative reaction rates of the successive inter- and
intramolecular nucleophilic attacks at the silicon atom, the reac-
tion of 1e with 2a was carried out in dry THF-d8 and monitored
by 1H NMR. The 1H signals of the resultant CF3CONHMe at 2.96
and 2.95 ppm (methyl group) and that of 3e at 5.17 ppm (5-H)
were used for the quantitation. As shown in Fig. 3, the analysis
showed that the first nucleophilic substitution, which was most
likely by the hydroxy group of 1e as depicted in Scheme 3, was
completed within 30 min, whereas the second substitution reac-
tion was significantly slow and required ca. 24 h, though the struc-
ture of the plausible reaction intermediate 8 could not be assigned
due to the complexity of the reaction mixture.
Secondly, we heated the stereo-enriched oxazasilolidines 3a–g,
which were configurationally stable at r.t., in dry toluene under ar-
gon to find that all of these compounds underwent epimerization
under these conditions as described above. It indicates that the
(RSi)- and (SSi)-isomers of 3a–g have similar thermodynamic stabil-
BnNHPh (2 equiv)
PhMeSi(NMeCOCF3)2
No reaction
< 5 min
CDCl3, rt, 2.5 h
2a
MeOH (2 equiv)
PhMeSi(OMe)2
quant
Scheme 2. 1H NMR analysis of reactivity of PhMeSi(NMeCOCF3)2 (2a) toward
model compounds of amino and hydroxy groups of 1,2-amino alcohols.
Fig. 3. Time course of formation of oxazasilolidine 3e and accompanying gener-
ation of N-methyl-2,2,2-trifluoroacetamide.
Fig. 4. Optimized structures and total energies of reaction intermediate and
product models calculated at the HF/6-31G level.
*

N. Oka et al. / Journal of Organometallic Chemistry 694 (2009) 2171–2178
2175
ity, and the preferential formation of the (RSi)-isomers of the oxaz-
4.2. Synthesis of bis(N-methyl-2,2,2-
asilolidine derivatives 3a–g was kinetically controlled. Ab initio
molecular orbital calculations also supported the kinetic control.
Thus, we used a model structure of the oxazasilolidine 3e, in which
the N-methyl-2,2,2-trifluoroacetamido group at the silicon atom
and the p-anisyl group at the amino group were replaced with an
N-methylacetamido group and a phenyl group, respectively, for
simplicity, and calculated the optimized structures and total ener-
gies of the two diastereomers [(2RSi,4S)- and (2SSi,4S)-3e0] and the
corresponding five-coordinated intermediates (I and II) (Fig. 4).
The calculations showed that (2SSi,4S)-3e0, the model for the minor
trifluoroacetamido)methylphenylsilane (2a)
N-Methyl-2,2,2-trifluoroacetamide (10.4 g, 82 mmol) was dis-
solved in dry THF (40 mL) under argon and dried over MS 4A over-
night. The solution was cooled to 0 °C and a 2.64 M solution of BuLi
in hexane (32 mL, 84 mmol) was added dropwise. The mixture was
then stirred at r.t. for 1 h, refluxed for 1 h, and cooled to r.t. Dichlo-
romethylphenylsilane (6.5 mL, 40 mmol) was then added drop-
wise, and the mixture was stirred for 2 h at 50 °C. The resultant
precipitate was removed by filtration through a glass filter under
argon, and the filtrate was concentrated under reduced pressure.
The residue was purified by distillation under reduced pressure
(bp 55–65 °C/30 Pa) to afford 2a (10.5 g, 28 mmol, 70%) as a color-
less liquid. A mixture of trifluoroacetamide rotamers. 1H NMR
(300 MHz, CDCl3) d 7.72–7.40 (5H, m), 3.14–2.97 (6H, m), 0.90,
0.85, 0.77 (3H, s, s, s). 13C NMR (75 MHz, CDCl3) d 141.9 (q,
isomer of 3e, was
E = 0.83 kcal/mol), while the intermediate I, which corresponds
to (2RSi,4S)-3e0, was significantly more stable than the intermedi-
ate II ( E = 2.21 kcal/mol), indicating that the major isomer of
a
little more stable than (2RSi,4S)-3e0
(D
D
the oxazasilolidine 3e was generated through an intermediate cor-
responding to I under kinetic conditions.
2
2JCF = 38.0 Hz), 140.7 (q, JCF = 38.9 Hz), 134.0, 133.4, 132.2, 131.6,
1
131.3, 130.3, 129.3, 128.6, 128.5, 128.4, 116.8 (q, JCF = 274 Hz),
3. Conclusion
1
1
116.7 (q, JCF = 273 Hz), 115.9 (q, JCF = 285 Hz), 115.6 (q,
1JCF = 284 Hz), 34.7, 34.5, ꢀ2.8, ꢀ3.1, ꢀ3.4. 29Si NMR (60 MHz,
CDCl3) d ꢀ13.3, ꢀ21.1. CI-MS: m/z 372 [M]+. ESI-HRMS: m/z calcd
for C13H15F6N2O2Si+ [(M+H)+] 373.0801, found 373.0828.
In conclusion, 1,3,2-oxazasilolidine derivatives proved to be
useful to synthesize Si-stereogenic tertiary silanes with excellent
optical purity through successive reactions with Grignard reagents
and a hydride reducing agent. A study on the oxazasilolidine ring
formation processes indicated that the oxazasilolidine rings were
formed via the fast nucleophilic attack of the hydroxy group of
1,2-amino alcohols and subsequent slow intramolecular cycliza-
tion by the nucleophilic attack of the amino group. The stereo-
chemical course of the ring formation was kinetically controlled.
Considering that the stereoselectivity of the reaction between the
oxazasilolidine and Grignard reagents was virtually independent
of the substituent introduced to the stereogenic silicon atom and
the resultant silyl ethers are expected to produce quaternary si-
lanes and other silyl ethers via subsequent reaction with C- and
O-nucleophiles, the method developed in this study would expand
the availability of optically active Si-stereogenic organosilicon
compounds.
4.3. Synthesis of (2RSi,4S)-2-methyl-3-(p-methoxyphenyl)-2,4-
diphenyl-1,3,2-oxazasilolidine [(2RSi,4S)-3e]. A general procedure for
the synthesis of 1,3,2-oxazasilolidine derivatives 3a–g
(S)-2-(p-Methoxyanilino)-2-phenylethanol [(S)-1e] (0.243 g,
1.0 mmol) was dried by repeated coevaporations with dry toluene
and dissolved in freshly distilled THF (2.0 mL) under argon. The
solution was slowly added dropwise to a stirred solution of 2a
(0.409 g, 1.1 mmol) in freshly distilled THF (3.0 mL) under argon
at r.t. The resultant mixture was stirred at r.t. under argon over-
night. The mixture was then concentrated to dryness under re-
duced pressure, and residual N-methyl-2,2,2-trifluoroacetamide
was removed in vacuo to afford crude 3e (0.328 g, 0.90 mmol,
90%). D.r. (RSi:SSi) of the crude 3e was 89:11 (1H NMR). The crude
product was washed with freshly distilled Et2O (3 ꢁ 1 mL) with a
4. Experimental
,4S)-3e
syringe under argon and dried in vacuo to afford (2RSi
(0.233 g, 0.64 mmol, 64%) as colorless crystals. D.r. (RSi:SSi) > 99:1.
1H NMR (300 MHz, CDCl3) d 7.63–7.19 (10H, m), 6.66–6.39 (4H,
m), 4.95 (1H, dd, J = 3.6, 6.3 Hz), 4.59 (1H, dd, J = 6.3, 9.3 Hz),
4.06 (1H, dd, J = 3.6, 9.9 Hz), 3.62 (3H, s), 0.92 (3H, s). 13C NMR
(75 MHz, CDCl3) d 152.0, 142.2, 139.2, 134.4, 134.2, 130.8, 128.7,
128.2, 127.2, 126.3, 116.9, 114.5, 72.2, 61.5, 55.4, ꢀ2.8. 29Si NMR
(60 MHz, CDCl3) d 4.4. CI-MS: m/z 361 [M]+. ESI-HRMS: m/z calcd
for C22H24NO2Si+ [(M + H)+] 362.1571, found 362.1575.
4.1. General information
All NMR spectra were recorded on a Varian Mercury 300. 1H
NMR spectra were obtained at 300 MHz with tetramethylsilane
(TMS) (d 0.0) as an internal standard in CDCl3 unless otherwise
noted. 13C NMR spectra were obtained at 75 MHz with CDCl3 as
an internal standard (d 77.0) in CDCl3. 29Si NMR spectra were ob-
tained at 60 MHz with tetramethylsilane (TMS) (d 0.0) as an inter-
nal standard in CDCl3. Chemical ionization (CI) mass spectra were
recorded on a Shimadzu GCMS-QP5050A mass spectrometer. ESI
mass spectra were recorded on an Applied Biosystems QSTAR.
4.4. Synthesis of methyl-1-naphthylphenyllsilyl ether 4a. A general
procedure for the synthesis of 4a–g
HPLC was carried out using
a
Daicel CHIRALCEL OD-H
(S)-2-(p-Methoxyanilino)-2-phenylethanol [(S)-1e] (0.243 g,
1.0 mmol) was dried by repeated coevaporations with dry toluene
and dissolved in freshly distilled THF (2.0 mL) under argon. The
solution was slowly added dropwise to a stirred solution of 2a
(0.409 g, 1.1 mmol) in freshly distilled THF (3.0 mL) under argon
at r.t. The resultant mixture was stirred at r.t. under argon over-
night. The mixture was then concentrated to dryness under re-
duced pressure, and residual N-methyl-2,2,2-trifluoroacetamide
was removed in vacuo to afford crude 3e, which was washed with
freshly distilled Et2O (3 ꢁ 1 mL) with a syringe under argon and
dried in vacuo to afford diastereopure 3e as colorless crystals.
The diastereopure 3e was dissolved in freshly distilled THF
(4.0 mL) under argon and the solution was cooled to ꢀ78 °C. A
freshly prepared solution of 1-naphthylmagnesium bromide in
(0.46 ꢁ 25 cm). The X-ray intensities were collected with a Rigaku
MERCURY CCD system and the crystal structure was solved with
the SHELXL-97 program [18]. Silica gel column chromatography
was carried out using Kanto silica gel 60 N (spherical, neutral,
63ꢀ210
lm) or NH silica gel (Fuji Silysia Chemical Inc., DM1020
100–200 mesh). Dry THF and ether were prepared by distillation
from sodium benzophenone ketyl under argon prior to use. Other
dry organic solvents were prepared by appropriate procedures.
The other organic solvents were reagent grade and used as re-
ceived. Ab initio molecular orbital calculations were carried out
using the Spartan’04 [19] on a Dell Inc. PRECISION 650 worksta-
tion. Geometry optimizations and single-point energy calculations
*
were carried out at the HF/6-31G level.

2176
N. Oka et al. / Journal of Organometallic Chemistry 694 (2009) 2171–2178
dry THF (0.5 M) (2.2 mL, 1.1 mmol) was added dropwise, and the
mixture was stirred for 2 h at ꢀ78 °C. Di-tert-butyl dicarbonate
(0.46 mL, 2.0 mmol) was added, and the mixture was allowed to
warm to r.t. and stirred for 1 h. A 0.3 M phosphate buffer solution
(pH 7.4) (10 mL) was then added, and the mixture was extracted
with Et2O (4 ꢁ 10 mL). The organic layers were combined and
washed with a 0.3 M phosphate buffer solution (pH 7.4) (10 mL).
The aqueous layers were combined and extracted with Et2O
(10 mL). The organic layers were combined, dried over Na2SO4, fil-
tered, and concentrated to dryness under reduced pressure. The
residue was purified by NH silica gel column chromatography
(20 g of NH silica gel, 0–1.5% AcOEt in hexane) to afford 4a
(0.325 g, 0.55 mmol, 55%) as a colorless foam. D.r. = 99:1. 1H
NMR (300 MHz, CDCl3) d 8.27 (1H, d, J = 8.4 Hz), 7.89 (1H, d,
J = 8.1 Hz), 7.84 (2H, t, J = 7.2 Hz), 7.47–7.08 (11H, m), 6.81–6.59
(6H, m), 4.65 (1H, t, J = 7.8 Hz), 4.34 (2H, m), 3.72 (3H, s), 1.41
(9H, s), 0.72 (3H, s). 13C NMR (75 MHz, CDCl3) d 157.1, 153.3,
139.7, 137.6, 137.1, 136.8, 135.3, 135.0, 134.3, 133.7, 133.4,
130.5, 129.2, 129.1, 128.8, 128.7, 127.7, 127.5, 127.3, 125.6,
125.4, 125.1, 113.4, 81.8, 66.8, 60.5, 55.3, 27.7, 0.4. 29Si NMR
(60 MHz, CDCl3) d ꢀ5.6. CI-MS: m/z 589 [M]+. ESI-HRMS: m/z calcd
for C37H40NO4Si+ [(M+H)+] 590.2721, found 590.2695. The enantio-
mer of 4a (0.354 g, 0.60 mmol, 60%) was also obtained as a color-
less foam from (R)-2-(p-methoxyanilino)-2-phenylethanol [(R)-
1e] (0.243 g, 1.0 mmol) according to the same procedure (Table
2, entry 2). D.r. = 1:99. NMR and CI-MS spectra were identical to
those of 4a. ESI-HRMS: m/z calcd for C37H40NO4Si+ [(M + H)+]
590.2721, found 590.2701.
m), 6.62–6.43 (5H, m), 6.10 (1H, dd, J = 3.6, 14.7 Hz), 5.76 (1H,
dd, J = 3.6, 20.4 Hz), 4.64 (1H, t, J = 7.5 Hz), 4.40–4.31 (2H, m),
3.72 (3H, s), 1.48 (9H, m), 0.35 (3H, s). 13C NMR (75 MHz, CDCl3)
d 157.0, 153.5, 139.6, 137.1, 137.0, 134.8, 134.0, 133.7, 129.3,
128.4, 127.8, 127.6, 127.1, 113.2, 81.9, 66.3, 59.7, 55.2, 27.8,
ꢀ2.8. 29Si NMR (60 MHz, CDCl3) d ꢀ10.0. CI-MS: m/z 489 [M]+.
ESI-HRMS: m/z calcd for C29H36NO4Si+ [(M+H)+] 490.2408, found
490.2412.
4.4.4. Allylmethylphenylsilyl ether (4e)
The silyl ether 4e (0.108 g, 0.21 mmol, 21%) was synthesized as
a
colorless oil from (S)-2-(p-methoxyanilino)-2-phenylethanol
[(S)-1e] (0.243 g, 1.0 mmol) according to the general procedure
shown above. D.r. = 94:6. 1H NMR (300 MHz, CDCl3) d 7.65–7.62
(2H, m), 7.42–7.35 (3H, m), 7.20–7.09 (3H, m), 6.70–6.50 (6H, m),
5.73 (1H, m), 4.88–4.80 (2H, m), 4.54 (1H, t, J = 7.5 Hz), 4.36–
4.24 (2H, m), 3.74 (3H, s), 1.95 (2H, m), 1.48 (9H, s), 0.28 (3H, s).
13C NMR (75 MHz, CDCl3) d 157.2, 153.5, 139.3, 137.2, 134.6,
134.5, 134.1, 129.4, 128.4, 127.7, 127.1, 114.0, 113.2, 81.9, 66.2,
59.5, 55.2, 27.7, 23.2, ꢀ3.6. 29Si NMR (60 MHz, CDCl3) d ꢀ3.7. CI-
MS: m/z 503 [M]+. ESI-HRMS: m/z calcd for C30H38NO4Si+
[(M+H)+] 504.2565, found 504.2542.
4.4.5. Methyl-2-methylpropen-1-ylphenylsilyl ether (4f)
The silyl ether 4f (0.306 g, 0.59 mmol, 59%) was synthesized as a
colorless oil from (S)-2-(p-methoxyanilino)-2-phenylethanol [(S)-
1e] (0.243 g, 1.0 mmol) according to the general procedure shown
above. D.r. = 96:4. 1H NMR (300 MHz, CDCl3) d 7.64–7.60 (2H, m),
7.34–7.28 (3H, m), 7.18–7.07 (3H, m), 6.72–6.69 (2H, m), 6.63–
6.53 (4H, m), 5.60 (1H, bs), 4.64 (1H, t, J = 7.2 Hz), 4.40–4.29
(2H, m), 3.72 (3H, s), 1.91 (3H, s), 1.67 (3H, s), 1.45 (9H, s), 0.35
(3H, s). 13C NMR (75 MHz, CDCl3) d 157.0, 154.5, 153.5, 139.7,
139.0, 135.4, 134.7, 133.8, 129.0, 128.5, 127.6, 127.6, 126.9,
121.2, 113.1, 81.7, 66.5, 59.5, 55.1, 29.7, 27.7, 23.9, ꢀ0.6. 29Si
NMR (60 MHz, CDCl3) d ꢀ13.0. CI-MS: m/z 517 [M]+. ESI-HRMS:
m/z calcd for C31H40NO4Si+ [(M+H)+] 518.2721, found 518.2725.
4.4.1. o-Anisylmethylphenylsilyl ether (4b)
The silyl ether 4b (0.310 g, 0.54 mmol, 54%) was synthesized as
a white foam from (S)-2-(p-methoxyanilino)-2-phenylethanol [(S)-
1e] (0.243 g, 1.0 mmol) according to the general procedure shown
above. The treatment of the oxazasilolidine 3e with o-anisyl Grig-
nard reagent was performed for 6 h. D.r. = 99:1. 1H NMR (300 MHz,
CDCl3) d 7.53 (2H, d, J = 6.3 Hz), 7.42–7.14 (8H, m), 6.96–6.82 (4H,
m), 6.68–6.55 (4H, m), 4.68 (1H, t, J = 7.5 Hz), 4.43–4.37 (2H, m),
3.76 (3H, s), 3.71 (3H, s), 1.37 (9H, s), 0.51 (3H, s). 13C NMR
(75 MHz, CDCl3) d 164.2, 156.8, 153.4, 140.0, 137.6, 137.6, 133.5,
131.5, 128.9, 128.9, 127.6, 127.3, 127.0, 124.5, 120.5, 113.0,
109.6, 81.6, 67.2, 59.9, 55.0, 54.6, 27.6, ꢀ0.7. 29Si NMR (60 MHz,
CDCl3) d ꢀ6.7. CI-MS: m/z 569 [M]+. ESI-HRMS: m/z calcd for
C34H40NO5Si+ [(M+H)+] 570.2670, found 570.2656.
4.4.6. Mesitylmethylphenylsilyl ether (4g)
The silyl ether 4g (0.248 g, 0.43 mmol, 43%) was synthesized as
a colorless foam from (S)-2-(p-methoxyanilino)-2-phenylethanol
[(S)-1e] (0.243 g, 1.0 mmol) according to the general procedure
shown above. D.r. = 95:5. 1H NMR (300 MHz, CDCl3) d 7.26–7.10
(8H, m), 6.88–6.79 (6H, m), 6.71–6.68 (2H, m), 4.62 (1H, t, J
= 7.5 Hz), 4.32 (2H, d, J = 7.8 Hz), 3.76 (3H, s), 2.27 (3H, s), 2.24
(6H, s), 1.40 (9H, s), 0.58 (3H, s). 13C NMR (75 MHz, CDCl3) d
157.3, 153.4, 145.8, 143.0, 139.7, 139.3, 135.3, 133.9, 133.8,
129.8, 129.4, 128.8, 128.1, 127.8, 127.5, 127.3, 113.5, 81.8, 69.6,
67.0, 60.8, 55.3, 27.7, 25.4, 21.0, 5.9. 29Si NMR (60 MHz, CDCl3) d
ꢀ8.2. CI-MS: m/z 581 [M]+. ESI-HRMS: m/z calcd for C36H44NO4Si+
[(M+H)+] 582.3034, found 582.3011.
4.4.2. Methylphenyl(o-tolyl)silyl ether (4c)
The silyl ether 4c (0.291 g, 0.53 mmol, 53%) was synthesized as
a white foam from (S)-2-(p-methoxyanilino)-2-phenylethanol [(S)-
1e] (0.243 g, 1.0 mmol) according to the general procedure shown
above. The treatment of the oxazasilolidine 3e with o-tolyl Grig-
nard reagent was performed for 6 h in dry THF–dry toluene (1:4,
v/v). D.r. = 99:1. 1H NMR (300 MHz, CDCl3) d 7.66–7.65 (1H, m),
7.64–7.13 (11H, m), 6.80–6.76 (4H, m), 6.65–6.62 (2H, m), 4.59
(1H, t, J = 7.2 Hz), 4.35 (2H, m), 3.72 (3H, s), 2.35 (3H, s), 1.42
(9H, s), 0.60 (3H, s). 13C NMR (75 MHz, CDCl3) d 156.9, 153.4,
144.4, 139.9, 138.0, 137.3, 135.2, 135.0, 133.4, 130.2, 129.8,
128.9, 128.6, 127.8, 127.4, 127.2, 124.8, 113.3, 81.8, 66.8, 60.3,
55.2, 27.7, 23.5, 0.2. 29Si NMR (60 MHz, CDCl3) d –5.8. CI-MS: m/z
553 [M]+. ESI-HRMS: m/z calcd for C34H40NO4Si+ [(M+H)+]
554.2721, found 554.2709.
4.5. Synthesis of (RSi)-methyl-1-naphthylphenyllsilane [(RSi)-5a]. A
general procedure for the synthesis of tertiary silanes 5a–c
Methyl-1-naphthylphenyllsilyl ether 4a (0.236 g, 0.40 mmol)
was dried by repeated coevaporations with dry toluene and dis-
solved in dry Bu2O (5.0 mL) under argon. A 1 M solution of diisobu-
tylaluminum hydride in hexane (2.8 mL, 2.8 mmol) was added
dropwise at r.t., and the mixture was stirred at 140 °C for 2 d.
The mixture was cooled to r.t. and diluted with Et2O (10 mL). A
1 M HCl aqueous solution (10 mL) was carefully added and the
mixture was extracted with Et2O (3 ꢁ 10 mL). The organic layers
were combined, washed with saturated NaCl aqueous solutions
(3 ꢁ 10 mL), dried over Na2SO4, filtered, and concentrated under
reduced pressure. The residue was purified by silica gel column
chromatography (15 g of silica gel, hexane) to afford (RSi)-5a
4.4.3. Methylphenylvinylsilyl ether (4d)
The silyl ether 4d (0.217 g, 0.44 mmol, 44%) was synthesized as
a
colorless oil from (S)-2-(p-methoxyanilino)-2-phenylethanol
[(S)-1e] (0.243 g, 1.0 mmol) according to the general procedure
shown above. D.r. = 96:4. 1H NMR (300 MHz, CDCl3) d 7.63–7.60
(2H, m), 7.38–7.31 (3H, m), 7.20–7.12 (3H, m), 6.81–6.77 (2H,

N. Oka et al. / Journal of Organometallic Chemistry 694 (2009) 2171–2178
2177
(37.2 mg, 0.15 mmol, 37%) as colorless crystals. 1H NMR spectrum
was identical to the data in the literature [8d]. Enantiomer ratio
was determined to be (RSi)-5a:(SSi)-5a = 98:2 by chiral HPLC anal-
ysis (Daicel CHIRALCEL OD-H, 0.46 ꢁ 25 cm, hexane, 0.4 mL/min).
(b) T. Kawashima, R.D. Kroshefsky, R.A. Kok, J.G. Verkade, J. Org. Chem. 43
(1978) 1111–1114.
[4] (a) R. Tacke, T. Kornek, T. Heinrich, C. Burschka, M. Penka, M. Pülm, C. Keim, E.
Mutschler, G. Lambrecht, J. Organomet. Chem. 640 (2001) 140–165;
(b) R. Tacke, D. Reichel, M. Kropfgans, P.G. Jones, E. Mutschler, J. Gross, X. Hou,
M. Waelbroeck, G. Lambrecht, Organometallics 14 (1995) 251–262.
[5] (a) Y. Kawakami, K. Takeyama, K. Komuro, O. Ooi, Macromolecules 31 (1998)
551–553;
4.5.1. (SSi)-Methyl-1-naphthylphenyllsilane ((SSi)-5a)
(SSi)-Methyl-1-naphthylphenyllsilane
0.16 mmol, 41%) was synthesized as colorless crystals from (SSi)-
(SSi)-5a
(40.4 mg,
(b) Y. Kawakami, K. Nakao, S. Shinke, I. Imae, Macromolecules 32 (1999)
6874–6876;
(c) K. Uenishi, I. Imae, E. Shirakawa, Y. Kawakami, Macromolecules 35 (2002)
2455–2460;
(d) Y. Kawakami, M. Omote, I. Imae, E. Shirakawa, Macromolecules 36 (2003)
7461–7468;
(e) K. Suzuki, Y. Kawakami, D. Velmurugan, T. Yamane, J. Org. Chem. 69 (2004)
5383–5389;
(f) A. Naka, J. Ikadai, J. Sakata, I. Miyahara, K. Hirotsu, M. Ishikawa,
Organometallics 23 (2004) 2397–2404;
(g) Y. Kakihana, K. Uenishi, I. Imae, Y. Kawakami, Macromolecules 38 (2005)
6321–6326.
methyl-1-naphthylphenyllsilyl
ether
[(SSi)-4a]
(0.236 g,
0.40 mmol) according to the general procedure shown above. 1H
NMR spectrum was identical to the data in the literature [8d].
Enantiomer ratio was determined to be (RSi)-5a:(SSi)-5a = 3:97 by
chiral HPLC analysis (Daicel CHIRALCEL OD-H, 0.46 ꢁ 25 cm, hex-
ane, 0.4 mL/min).
4.5.2. o-Anisylmethylphenylsilane (5b)
[6] (a) L.H. Sommer, C.L. Frye, G.A. Parker, K.W. Michael, J. Am. Chem. Soc. 86
(1964) 3271–3276;
o-Anisylmethylphenylsilane 5b (31.2 mg, 0.14 mmol, 68%) was
synthesized as a colorless liquid from the o-anisylmethylphenylsi-
lyl ether 4b (0.114 g, 0.20 mmol) according to the general proce-
dure shown above. 1H NMR spectrum was identical to the data
in the literature [20]. Enantiomer ratio was determined to be
(RSi)-5b:(SSi)-5b = 97:3 by chiral HPLC analysis (Daicel CHIRALCEL
OD-H, 0.46 ꢁ 25 cm, hexane, 0.4 mL/min). The assignment of the
absolute configuration is tentative.
(b) L.H. Sommer, C.L. Frye, J. Am. Chem. Soc. 81 (1959) 1013.
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(1982) 924–927;
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4.5.3. Methylphenyl(o-tolyl)silane (5)
Methylphenyl(o-tolyl)silane 5c (22.9 mg, 0.11 mmol, 54%) was
synthesized as a colorless liquid from the methylphenyl(o-tolyl)si-
lyl ether 5c (0.111 g, 0.20 mmol) according to the general proce-
dure shown above. 1H NMR spectrum was identical to the data
in the literature [20]. Enantiomer ratio could not be determined
by chiral HPLC analysis.
(b) A. Nakazaki, J. Usuki, K. Tomooka, Synlett. (2008) 2064–2068;
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(f) J.F. Klebe, H. Finkbeiner, J. Am. Chem. Soc. 90 (1968) 7255–7261;
(g) J.F. Klebe, H. Finkbeiner, J. Am. Chem. Soc. 88 (1966) 4740–4741;
(h) W.J. Richter, J. Organomet. Chem. 169 (1979) 9–17;
(i) R.J.P. Corriu, J.J.E. Moreau, J. Organomet. Chem. 120 (1976) 337–346;
(j) R.J.P. Corriu, J.J.E. Moreau, J. Organomet. Chem. 85 (1975) 19–33;
(k) T. Hayashi, K. Yamamoto, M. Kumada, Tetrahedron Lett. 15 (1974) 331–
334;
5. Supplementary material
CCDC 701477 contains the supplementary crystallographic data
for (2RSi,4S)-3e. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(l) R.J.P. Corriu, J.J.E. Moreau, Tetrahedron Lett. 14 (1973) 4469–4473.
[9] M.A. Brook, Silicon in Organic Organometallic and Polymer Chemistry, John
Wiley and Sons, New York, 2000. pp. 27–38.
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(b) L.H. Sommer, J.E. Lyons, H. Fujimoto, J. Am. Chem. Soc. 91 (1969) 7051–7061;
(c) L.H. Sommer, J.E. Lyons, J. Am. Chem. Soc. 91 (1969) 7061–7067;
(d) L.H. Sommer, K.W. Michael, H. Fujimoto, J. Am. Chem. Soc. 89 (1967) 1519–
1521;
(e) L.H. Sommer, J.E. Lyons, J. Am. Chem. Soc. 89 (1967) 1521–1522.
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(b) O.I. Kolodiazhnyi, Tetrahedron: Asymmetry 9 (1998) 1279–1332;
(c) R.P. Iyer, M.-J. Guo, D. Yu, S. Agrawal, Tetrahedron Lett. 39 (1998) 2491–
2494;
Acknowledgments
We thank Professor Kazuhiko Saigo (University of Tokyo) for
helpful suggestions. We also thank Professor Yuka Kobayashi
(Waseda University) for X-ray crystallographic analysis. This re-
search was supported by grants from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) Japan.
(d) R.P. Iyer, D. Yu, N.-H. Ho, W. Tan, S. Agrawal, Tetrahedron: Asymmetry 6
(1995) 1051–1054;
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(f) D.B. Cooper, J.M. Harrison, T.D. Inch, Tetrahedron Lett. 15 (1974) 2697–
2700.
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under argon to epimerize. The resultant oxazasilolidine 3e with a lower d.r.
(RSi:SSi = ca. 80:20) was used to synthesize silyl ethers 4a–g, which were
obtained with d.r. of 72:28–84:16.
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