Synthesis of [2.2]paracyclophane-based bidentate oxazoline–carbene ligands for the asymmetric 1,2-silylation of N-tosylaldimines

Article abstract of DOI:10.1016/j.tetasy.2017.05.001

A series of novel oxazoline-substituted imidazolium salts with planar and central chirality has been successfully synthesized and applied to copper-catalyzed enantioselective 1,2-silylation of N-tosylaldimines. The oxazoline–carbene copper complex generated in situ by the reaction of the oxazoline-substituted imidazolium and Cu₂O demonstrated an exceptionally high catalytic activity in the asymmetric 1,2-silylation of N-tosylaldimines, affording chiral α-amino silanes with excellent yields and enantioselectivities.

Full text of DOI:10.1016/j.tetasy.2017.05.001

Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Synthesis of [2.2]paracyclophane-based bidentate oxazoline–carbene
ligands for the asymmetric 1,2-silylation of N-tosylaldimines
a,
Xichao Wang a, Zhen Chen c, Wenzeng Duan a, Chun Song b, , Yudao Ma
⇑
⇑
a Department of Chemistry, Shandong University, Shanda South Road No. 27, Jinan 250100, PR China
b School of Pharmaceutical Sciences, Shandong University, West Wenhua Road No. 44, Jinan 250012, PR China
c School of Chemistry and Chemical Engineering, Hebei Normal University For Nationalities, Higher Education Park, Chengde 0670000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel oxazoline-substituted imidazolium salts with planar and central chirality has been suc-
cessfully synthesized and applied to copper-catalyzed enantioselective 1,2-silylation of N-tosylaldimines.
The oxazoline–carbene copper complex generated in situ by the reaction of the oxazoline-substituted
imidazolium and Cu2O demonstrated an exceptionally high catalytic activity in the asymmetric 1,2-sily-
Received 8 March 2017
Revised 18 April 2017
Accepted 27 April 2017
Available online xxxx
lation of N-tosylaldimines, affording chiral
a
-amino silanes with excellent yields and enantioselectivities.
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
Cu–secondary diamine complex.9 Despite many excellent results
being achieved, the design of novel chiral ligands to enhance the
enantioselectivity remains a major focus.
Organosilicons are versatile building blocks in organic chem-
istry. The CASi bond, once formed, can be converted into various
functional groups.1 A convenient synthetic approach to this class
of compounds employs the catalytic addition of Si nucleophiles,
generated from reactive silyl-boronate reagents, such as PhMe2-
Si-Bpin and PhCl2Si-SiMe3,2 to unsaturated acceptors. While enan-
Planar chiral [2,2]paracyclophane-based ligands have received
much attention over the past few decades due to their utility in
asymmetric catalysis.10 Since Bolm et al. reported on the first car-
bene precursors derived from [2.2]paracyclophane,11 much pro-
gress has been made with respect to the synthesis and the
catalytic applications of planar chiral [2.2]paracyclophane-based
carbene precursors and their metal complexes. In 2011, our group
successfully synthesized a planar chiral pseudo-ortho-disubsti-
tuted [2.2]paracyclophanyl bidentate oxazoline–carbene ligand
(S,SP)-1 (Fig. 1) and applied it to the Cu-catalyzed asymmetric b-
tioselective 1,4-silylations of
a,b-unsaturated carbonyl and
carboxyl compounds have already been widely studied,3 asymmet-
ric 1,2-silylations of imines have attracted increasing attention in
recent years, particularly due to the potential application of the
resultant chiral
a-amino silanes in silicon-containing peptide iso-
steres and
a
-amino acids.4 In 2011, a catalytic system of CuCN/
boration of a,b-unsaturated ketones, although the enantioselectiv-
NaOMe/MeOH was applied to catalyze the racemic 1,2-silylation
of aldimines and ketimines using PhMe2Si-Bpin as the silylation
reagent.5 Since then, investigations have been focused on the
asymmetric version. In 2014, Oestreich et al. disclosed the first
enantioselective 1,2-silylation of various protected imines cat-
alyzed by a chiral six-membered carbene–Cu(I) complex.6 Then
He et al. reported the Cu(I)-catalyzed asymmetric 1,2-silylation
of N-tosylaldimine by utilizing Hoveyda’s chiral imidazolinium
salts as ligands.7 Very recently, our group synthesized a series of
novel chiral bicyclic 1,2,4-triazolium salts and applied an optimal
carbene precursor as an organocatalyst and a ligand for copper
catalyst in the same reaction.8 Mita and Sato reported on the
enantioselective silylation of N-tert-butylsulfonylimines using a
ities of the addition products were only moderate.12 Subsequently,
we prepared L-tert-leucinol derived oxazoline–carbene precursors
(S,SP)-2 and (S,SP)-3 for the Cu(I)-catalyzed asymmetric b-boration
of
a,b-unsaturated esters, noting the important influence of the
substituents on both the pyridinium and the oxazoline ring on
the enantioselectivity of the reaction.13 Based on these observa-
tions, we wanted to further modify the structure of [2.2]paracyclo-
phanyl oxazoline–carbene ligand by introducing
a range of
substituents at either the pyridinium or the oxazoline ring exhibit-
ing different steric effects in order to evaluate the effect of sub-
stituents on the catalytic behavior of the relevant complex. To
examine the effects of such a modulation on the catalytic perfor-
mances of the resulting complex, we herein report the synthesis
of three novel oxazoline-substituted [2.2]paracyclophane-based
imidazolium salts with planar and central chirality and their
⇑
Corresponding authors. Tel.: +86 531 883 61869; fax: +86 531 883 64464.
E-mail addresses: chunsong@sdu.edu.cn (C. Song), ydma@sdu.edu.cn (Y. Ma).
http://dx.doi.org/10.1016/j.tetasy.2017.05.001
0957-4166/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, X.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.05.001

2
X. Wang et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
N
N
N
N
N
N
N
N
N
Cl
OTf
Cl
O
O
O
(S,SP)-1
(S,SP)-2
(S,SP)-3
Figure 1. Structures of the reported bidentate oxazoline–carbene precursors.
NH
application in the Cu(I)-catalyzed enantioselective 1,2-silylation of
Ph
Ph
N-tosylaldimines.
Br
Br
Pd-DPPF
N
COOH
2. Results and discussion
NaOBu-t/toluene
O
On the basis of our previously reported L-tert-leucinol derived
oxazoline–carbene precursors (S,SP)-3, we designed and synthe-
7
8
8
(S,RP)- and (S,SP)-
inseparable
sized three new [2.2]paracyclophanyl oxazoline–carbene precur-
Ph
sors derived from
selected as the substituent on the oxazoline ring because
is less expensive than -tert-leucinol and the size of the iso-propyl
L-valinol (Fig. 2). The iso-propyl group was
O
Ph
L-valinol
N
N
N
N
recrystallization
HCl aq
L
group is large enough for resolution and construction of an effec-
tive chiral environment. On the other hand, we anticipated that
the simple introduction of a range of substituents at pyridine ring
might allow for rapid and effective fine-tuning of the steric proper-
ties of the carbene center to enhance its chiral recognition ability.
As shown in Scheme 1, the synthesis of the imidazolium salts
started from 4-bromo-12-oxazolinyl[2.2]paracyclophane 8, which
can be obtained from racemic 4-bromo-12-carboxyl [2.2]paracy-
clophane 7 following the literature reported by Bolm et al.14 How-
ever, the two diastereoisomers could not be separated at this step.
The reaction of 8 with benzhydrylideneamine under Pd-catalyzed
amination, afforded the corresponding imines (S,RP)-9 and (S,SP)-
9 in an overall yield of 85%.15 The desired enantiopure (S,SP)-9
was easily separated from the mixture of diastereoisomers by dou-
ble recrystallization with ethanol rather than repeated column
chromatography on silica gel. 4-Amino-12-oxazolinyl[2.2]paracy-
clophane (S,SP)-10 was then obtained through acid hydrolysis of
(S,SP)-9 with aqueous HCl under mild conditions in 88% yield.
Treatment of (S,SP)-9 with 6-methylpyridine-2-carbaldehyde in
dichloromethane, using 4 Å molecular sieves as the dehydrating
agent, afforded the corresponding imine 11a, which was not stable
enough for further purification and used directly. Imine 11a was
then reacted with a reagent formed from silver trifluoromethane-
sulfonate and chloromethyl pivalate, resulted in the formation of
the imidazolium triflate. Finally, the desired imidazolium chloride
(S,SP)-4 was obtained by anion exchange of its analogue triflate
with an ion-exchange resin according to the same method reported
by McQuade et al.16 Following the same procedure as above, (S,SP)-
5 and (S,SP)-6 were obtained from 6-cyclopropylpyridine-2-car-
baldehyde and quinoline-2-carbaldehyde, respectively (Scheme 2).
The absolute configuration of these imidazolium chlorides was
determined by comparing the specific rotation of 12 prepared by
THF
Ph
O
Ph
9
(S,SP)-
(S,RP)-9 and (S,SP)-9
NH2
6-methylpyridine-2-carbaldehyde or
6-cyclopropylpyridine-2-carbaldehyde or
2-quinolinecarboxaldehyde
11a
11b
11c
N
O
Molecular sieves, CH2Cl2, 40 R
&
10
(S,SP)-
Scheme 1. Synthesis of enantiopure imines 11a–c.
acid hydrolysis of (S,SP)-10 with the reported literature value
(Scheme 3).17
With these NHC precursors in hand, the corresponding [2.2]-
paracyclophanyl oxazoline–carbene copper(I) complexes were
generated in situ by the reaction of the imidazolium salts and
Cu2O in THF at 60 °C overnight. According to our previously
reported procedure,8 their catalytic performances were screened
using N-tosylaldimine 13a as model substrate and Me2PhSi-B
(pin) as a silylation reagent in toluene at 0 °C for 15 min. As shown
in Table 1, all carbene–Cu complexes exhibited good catalytic
activity, affording the addition product in yields of 90–95%. Nota-
bly, changing the substituent on the oxazoline ring from tert-butyl
to less bulkier iso-propyl did not affect the enantioselectivity
(Table 1, entries 1–2), but the yield had a slight increase (from
90% to 94%). Then, the effect of substituent at the
a-position of
pyridinium on enantioselectivity was also examined. Unfortu-
nately, increasing the steric hindrance resulted in a large decrease
in enantioselectivity (Table 1, entries 3–4). We ascribe the lower ee
N
N
N
N
N
Cl
N
N
Cl
N
Cl
N
O
O
O
4
5
6
(S,SP)-
(S,SP)-
(S,SP)-
Figure 2. Structures of new bidentate oxazoline–carbene precursors.
Please cite this article in press as: Wang, X.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.05.001

X. Wang et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
3
In an attempt to improve the catalytic enantioselectivity, we
therefore examined several reaction conditions (Table 2). The reac-
tion did not proceed well in MTBE and DCE (Table 2, entries 1–2),
and a sharp decrease in the enantioselectivity was observed
(Table 2, entry 2). The use of DME or EA did not affect the reaction
rate, but the ee values were not improved (Table 2, entries 3–4).
Among the solvents screened, toluene was still the optimal solvent
in terms of yield and stereoselectivity (Table 1, entry 2). The impact
of temperature on the silylation reaction was evaluated. When the
reaction temperature was increased from 0 to 20 °C, both the yield
and enantioselectivity were decreased (Table 2, entry 5). However,
lowering the reaction temperature from 0 to À20 °C did not alter
the enantiomeric excess of 14a by any appreciable amount (Table 2,
entry 6). The effect of the base was also tested (Table 2, entries 7–
8). The enantioselectivity was not enhanced by using NaOBu-t
instead of NaOMe (Table 2, entry 7). The desired silylation did
not occur when LiOMe was used as the base (Table 2, entry 8).
Under the optimal reaction conditions, the substrate scope was
investigated with a variety of N-tosylaldimines. It was found that
substituents at the ortho- or para-position in the benzene ring of
N-tosylaldimines had little effect on the yield or enantioselectivity,
regardless of whether it is an electron-donating substituent or an
electron-withdrawing one, providing the corresponding addition
products with excellent yields and enantioselectivities (Table 3,
entries 2–7 and 11–13). Moreover, the substrates bearing a sub-
stituent at the meta-position in the phenyl ring, no matter whether
it was an electron-donating substituent or an electron-withdraw-
ing one, gave the desired products in excellent yields albeit the
ee values were somewhat lower (Table 3, entries 8–10). Further-
more, 1- and 2-naphthyl substituted N-tosylaldimines were also
compatible with the reaction conditions and afforded the corre-
sponding adducts in good yields and with high enantiomeric
excesses (Table 3, entries 14–15). However, a heteroaromatic thio-
phene-substituted N-tosylaldimine was not a good substrate, and a
lower enantioselectivity was observed (Table 3, entry 16). The
same protocol also allowed for the enantioselective 1,2-silylation
of aliphatic N-tosylaldimines, high yield and enantioselectivity
could be obtained (Table 3, entry 17).
1. AgOTf
N
N
N
ClCH2OCOtBu
N
N
N
Cl
2. anion-exchange
resin
O
O
(S,SP)-4
11a
N
N
N
1. AgOTf
ClCH2OCOtBu
N
N
N
Cl
2. anion-exchange
resin
O
O
11b
(S,SP)-5
1. AgOTf
ClCH2OCOtBu
N
N
N
N
N
Cl
N
2. anion-exchange
resin
O
O
11c
6
(S,SP)-
Scheme 2. Synthesis of the imidazolium chlorides.
NH2
N
NH2
HCl aq
refluxed for 6 h
COOH
O
12
10
(S,SP)-
Scheme 3. Synthesis of enantiopure pseudo-ortho amino acid.
values obtained from the (S,SP)-5 and (S,SP)-6 to their structures, in
which the substituents are so bulky as to inhibit the construction
of an effective chiral environment. Furthermore, it revealed that
the size of the substituent at pyridine ring plays a key role in the
asymmetric catalysis. On the other hand, in order to study the role
of the oxazoline unit at [2.2]paracyclophane backbone, we synthe-
sized unsubstituted [2.2]paracyclophane-based carbene precursor
(SP)-16 (Scheme 4). This monodentate carbene ligand showed good
catalytic activity but lower enantioselectivity, indicating that the
oxazoline unit is vital to create an excellent chiral environment
(Table 1, entry 5). Based on these results, (S,SP)-4 was selected as
a suitable ligand for further investigation.
3. Conclusions
In conclusion, a series of new [2.2]paracyclophanyl oxazoline–
carbene precursors derived from L-valinol were synthesized and
successfully applied to Cu-catalyzed asymmetric 1,2-silylations of
N-tosylaldimines. It was found that changing the substituent at
the oxazoline ring on the [2.2]paracyclophanyl oxazoline–carbene
backbone from tert-butyl to less bulkier iso-propyl did not affect
Table 1
Evaluation of different NHC ligandsa
Cu2O (2.5 mol%)
Ts
Ts
H
O
O
ligand (5 mol%)
HN
N
Ph
Si B
+
NaOMe (1.5 eq)
Ph
SiMe2Ph
Ph
R
&
toluene, 0
13a
14a
Entry
Ligand
Time (min)
Yield (%)b
ee (%)c
1
2
3
4
5
(S,SP)-3
(S,SP)-4
(S,SP)-5
(S,SP)-6
(SP)-16
15
15
15
15
15
90
95
92
95
90
96
96
88
82
83
a
b
c
The reaction was carried out with ligand (5 mol %), Cu2O (2.5 mol %), base (1.5 equiv), PhMe2Si-Bpin (1.5 equiv) and 13a (0.1 mmol) in toluene (2.0 mL) at 0 °C for 15 min.
Yield of the isolated product.
Determined by chiral HPLC (CHIRALPAK IA) analysis.
Please cite this article in press as: Wang, X.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.05.001

4
X. Wang et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
1. AgOTf
ClCH2OCOtBu
N
NH2
N
N
CHO
Cl
2. anion-exchange
resin
(SP)-15
(SP)-16
Scheme 4. Synthesis of monodentate carbene ligand.
Table 2
Optimization of reaction conditionsa
Cu2O (2.5 mol%)
O
Ts
Ts
H
(S,SP)-4 (5 mol%)
HN
N
Ph
Si B
+
base (1.5 equiv)
O
SiMe2Ph
Ph
Ph
solvent, T R
&
13a
14a
Entry
Solvent
T (°C)
Base
Yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
MTBE
DCE
DME
0
0
0
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOtBu
LiOMe
76
85
92
90
88
92
90
<5
94
32
93
82
93
96
90
n.d.
EA
0
Toluene
Toluene
Toluene
Toluene
25
À20
0
0
a
The reaction was carried out with (S,SP)-4 (5 mol %), Cu2O (2.5 mol %), base (1.5 equiv), PhMe2Si-Bpin (1.5 equiv) and 13a (0.1 mmol) in solvent (2.0 mL) at indicated
temperature for 15 min.
b
Yield of the isolated product.
Determined by chiral HPLC (CHIRALPAK IB) analysis.
c
Table 3
Scope of the methodologya
Cu2O (2.5 mol%)
(S,SP)-4 (5 mol%)
Ts
Ts
N
O
O
HN
Ph
Si B
+
R
SiMe2Ph
NaOMe (1.5 equiv)
R
H
R
toluene, 0
&
14
13
Entry
Aldimines
R
Yield (%)b
ee (%)c
1
2
3
4
13a
13b
13c
13d
13e
13f
13g
13h
13i
13j
13k
13l
13m
13n
13o
13p
13q
Ph
p-MeC6H4
p-MeOC6H4
p-CF3C6H4
p-FC6H4
95 14a
90 14b
93 14c
96 14d
98 14e
98 14f
97 14g
97 14h
96 14i
97 14j
96 14k
94 14l
96 14m
94 14n
95 14o
90 14p
90 14q
96
95
95
96
97
95
94
86
87
91
95
95
92
94
94
86
94
5
6
p-ClC6H4
p-BrC6H4
7d
8
m-MeC6H4
m-MeOC6H4
m-CF3C6H4
o-MeC6H4
o-MeOC6H4
o-CF3C6H4
1-Naphthyl
2-Naphthyl
2-Thienyl
Cyclohexyl
9
10
11
12
13
14
15
16
17
a
The reaction was carried out with (S,SP)-4 (5 mol %), Cu2O (2.5 mol %), NaOMe (1.5 equiv), PhMe2Si-Bpin (1.5 equiv) and 13 (0.1 mmol) in toluene (2.0 mL) at 0 °C for
15 min.
b
Yield of the isolated product.
Determined by chiral HPLC (CHIRALPAK IB) analysis.
Toluene (3.0 mL) was used as solvent.
c
d
the catalytic behavior of relevant complex. In contrast, the size of
the substituent at pyridine ring on the carbene skeleton had a sig-
nificant influence on the catalytic performances of the resulting
complex. Moreover, due to inhibit the free rotation about CAN
bond connecting the carbene ring and [2.2]paracyclophane moiety,
the conformation of copper complex with oxazoline–carbene
bidentate ligand (S,SP)-4 is more stable than that with monoden-
tate carbene ligand.8 Thus, the coordination of the oxazoline side-
arm to the metal center improves the stability, activity, and chiral
environment of the catalyst. Furthermore, the screening of ligands
Please cite this article in press as: Wang, X.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.05.001

X. Wang et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
5
reveals that (S,SP)-4 was the optimal ligand in terms of enantiose-
lectivity and yield. The corresponding chiral -amino silanes could
be obtained in excellent yields (up to 98%) with high enantioselec-
9–11. The mixture was then extracted with CH2Cl2 (3 Â 5.0 ml).
The combined organic layers were dried over anhydrous Na2SO4,
filtered and concentrated in vacuo. The residue was purified by col-
umn chromatography (petroleum ether/ethyl acetate = 5/1) to
afford (S,SP)-10 as a white solid. Yield: 475 mg (89% yield); mp:
a
tivities (up to 97% ee).
136–138 °C; [
a
]
D
20 = À1.5 (c 1.0, CH2Cl2); 1H NMR (300 MHz, CDCl3)
4. Experimental
4.1. General
d 8.06–7.53 (m, 1H), 7.26 (s, 1H), 6.72–6.41 (m, 2H), 6.39–6.02 (m,
2H), 5.62–5.39 (m, 1H), 4.45–4.26 (m, 1H), 4.21–3.97 (m, 3H), 3.50
(s, 2H), 3.25–3.00 (m, 3H), 3.00–2.74 (m, 3H), 2.75–2.56 (m, 1H),
2.11–1.74 (m, 1H), 1.12 (d, J = 6.7 Hz, 3H), 1.03 (d, J = 6.7 Hz, 3H).
13C NMR (75 MHz, CDCl3) d 163.9, 145.6, 141.5, 140.0, 138.8,
135.4, 135.1, 135.0, 128.2, 128.2, 124.3, 122.7, 120.5, 72.8, 69.6,
34.9, 34.2, 33.3, 32.4, 32.1, 19.1, 18.6. HRMS (ESI) m/z calcd for
[M+H]+ (C22H26N2O): 335.2123, found: 335.2122.
Commercially available reagents were used without further
purification unless otherwise noted. Solvents were reagent grade
and purified by standard techniques. Melting points were recorded
on a melting point apparatus and were uncorrected. Optical rota-
tions were taken on a polarimeter with a wavelength of 589 nm.
Mass The concentration ‘c’ has units of g/100 ml (or 10 mg/ml)
unless otherwise noted. 1H NMR and 13C NMR spectra were recorded
on a Bruker AVANCE-300 and AVANCE-400 spectrometer at 298 K.
Chemical shifts are reported in parts per million (ppm) downfield
from tetramethylsilane (TMS) with reference to the internal solvent
for 1H NMR and 13C NMR spectra. Mass spectra were recorded on an
Agilent Technologies 6510 Q-Tof LC/MS. Enantiomeric excess was
determined by HPLC on a Chiralpak IB chiral column.
4.2.3. Imidazolium salt (S,SP)-4
An oven-dried schlenk flask was charged with (S,SP)-10
(120 mg, 0.36 mmol), 6-methyl-2-pyridine aldehyde (43.6 mg,
0.36 mmol) and 4 Å molecular sieves (500 mg) in CH2Cl2
(1.5 mL). The reaction mixture was stirred vigorously at 40 °C for
48 h. After filtration over Celite, the collected organic layers were
evaporated under reduced pressure to afford the corresponding
imine 11a, which was used directly without further purification.
To a solution of AgOTf (149 mg, 0.58 mmol) in THF (0.5 mL) was
4.2. General procedures for the synthesis of imidazolium
chlorides (S,SP)-4, (S,SP)-5 and (S,SP)-6
added chloromethyl pivalate (77.8 lL, 0.54 mmol) and the mixture
was stirred in a sealed tube in the dark at room temperature for
10 min, during which time, a white precipitate appeared. The sus-
pension was filtered over Celite and washed with dry CH2Cl2
(2.0 ml). The filtrate was then added to the imine 11a, and the mix-
ture was stirred in a sealed tube in the dark at 40 °C for 24 h. After
completion of the reaction (monitored by TLC), the mixture was
allowed to cool to room temperature, and ethanol (2.0 ml) was
added. After filtration over Celite, the solvent was evaporated
under reduced pressure and the residue was purified by column
chromatography (CH2Cl2/ethanol = 30:1) to afford the desired imi-
dazo[1,5-a]pyridinium triflate as a white solid. The corresponding
imidazo[1,5-a]pyridinium chloride (S,SP)-4 was prepared by anion
exchange of its triflate analogue with an ion-exchange resin (chlo-
ride form), following the protocol developed by McQuade.16 (S,SP)-
4 was obtained as a white solid. Yield: 90 mg (51%); mp: 246–
4.2.1. (S,SP)-4-Benzhydrylideneamino-12-(4-isopropyloxazolin-
2-yl)[2.2]paracyclophane (S,SP)-9
Under nitrogen, an oven-dried schlenk flask was charged with
diastereomeric mixture of 4-bromo-12-(4-isopropyloxazolin-2-yl)
[2.2]paracyclophane
(2.0 g,
5 mmol),
Pd-DPPF
(122 mg,
0.15 mmol), benzhydrylideneamine (1.3 ml, 7.5 mmol), NaOtBu
(720 mg, 7.5 mmol) and toluene (10.0 mL). The resulting mixture
was heated at reflux for 16 h. After completion of the reaction
(monitored by TLC), the solution was allowed to cool to room tem-
perature and then quenched with H2O (10.0 ml). The organic layer
was separated and the aqueous phase was extracted with CH2Cl2
(3 Â 10.0 ml). The combined organic layers evaporated to dryness
under reduced pressure and the residue was purified by column
chromatography (petroleum ether/ethyl acetate = 20/1) to afford
the (S,RP)-9 and (S,SP)-9 in an overall yield of 85% (2.12 g). Enan-
tiomerically pure (S,SP)-9 was easily separated from the mixture
of diastereoisomers by double recrystallization with ethanol as a
yellow solid. Yield: 800 mg (64% yield); mp: 168–170 °C;
248 °C; [
a]
20 = À225.6 (c 0.1, CH2Cl2); 1H NMR (300 MHz, CDCl3)
D
d 11.01 (s, 1H), 9.16 (s, 1H), 7.98–7.44 (m, 1H), 7.20–6.99 (m,
3H), 6.86 6.91–6.82 (m, 1H), 6.79–6.59 (m, 4H), 4.74–4.56 (m,
1H), 4.53–4.37 (m, 1H), 4.37–4.12 (m, 1H), 4.12–3.94 (m, 1H),
3.65–3.46 (m, 1H), 3.30–2.62 (m, 8H), 2.42–2.13 (m, 1H), 2.02–
1.81 (m, 1H), 1.06 (d, J = 6.7 Hz, 3H), 0.95 (d, J = 6.7 Hz, 3H). 13C
NMR (75 MHz, CDCl3) d 165.1, 142.5, 140.9, 139.4, 137.5, 136.2,
135.0, 134.3, 134.0, 133.1, 130.9, 130.2, 128.3, 126.5, 125.8,
125.2, 116.7, 116.3, 114.6, 71.4, 70.6, 35.9, 34.0, 33.6, 33.3, 32.8,
19.6, 18.9. HRMS (ESI) m/z calcd for [MÀCl]+ (C30H32N3O):
450.2545, found: 450.2539.
[
a]
D
20 = À742.8 (c 0.2, CH2Cl2); 1H NMR (300 MHz, CDCl3) d 7.95
(s, 1H), 7.93–7.87 (m, 2H), 7.59–7.38 (m, 3H), 7.20–7.10 (m, 3H),
7.03–6.93 (m, 2H), 6.62–6.57 (m, 1H), 6.27–6.40 (m, 2H), 5.29–
5.27 (m, 1H), 4.43–4.16 (m, 2H), 4.12–3.90 (m, 2H), 3.57–3.32
(m, 2H), 3.18–3.00 (m, 1H), 2.94–2.67 (m, 3H), 2.50–2.62 (m,
1H), 1.96–1.71 (m, 1H), 1.04 (d, J = 6.7 Hz, 3H), 0.95 (d, J = 6.7 Hz,
3H). 13C NMR (75 MHz, CDCl3) d 164.4, 163.2, 148.5, 140.7, 140.6,
140.3, 140.0, 136.7, 135.3, 135.0, 133.9, 132.8, 131.0, 130.2,
129.6, 129.5, 128.3, 128.1, 128.0, 127.7, 127.6, 124.2, 73.3, 69.0,
35.3, 33.7, 33.4, 33.2, 32.9, 19.2, 18.8. HRMS (ESI) m/z calcd for
[M+H]+ (C35H34N2O): 499.2749, found: 499.2750.
4.2.4. Imidazolium salt (S,SP)-5
(S,SP)-5 was prepared by the same procedure as for (S,SP)-4,
using 6-cyclopropylpyridine-2-carbaldehyde instead of 6-methyl-
pyridine-2-carbaldehyde. The title compound was obtained as a
white solid. Yield: 83 mg (45%); mp: 246–248 °C; [
a
]
20 = À225.6
D
(c 0.1, CH2Cl2); 1H NMR (300 MHz, CDCl3) d 11.47 (s, 1H), 9.76 (s,
1H), 7.78–7.60 (m, 1H), 7.22–7.11 (m, 2H), 6.96–6.65 (m, 7H),
4.61–4.48 (m, 1H), 4.44–4.17 (m, 2H), 4.10–3.89 (m, 1H), 3.86–
3.66 (m, 1H), 3.37–2.98 (m, 5H), 3.03–2.66 (m, 3H), 2.48–2.21
(m, 1H), 2.03–1.68 (m, 2H), 1.16 (d, J = 6.6 Hz, 3H), 1.06 (d,
J = 6.6 Hz, 3H). 13C NMR (75 MHz, CDCl3) d 165.4, 141.8, 140.6,
139.9, 139.7, 137.9, 136.2, 135.2, 135.1, 134.1, 133.8, 131.0,
130.4, 128.3, 126.4, 125.9, 125.3, 115.9, 114.7, 112.9, 72.0, 70.8,
4.2.2. (S,SP)-4-Amino-12-(4-isopropyloxazolin-2-yl)[2.2]para-
cyclophane (S,SP)-10
To
a
solution of (S,Sp)-4-benzhydrylideneamino-12-(4-iso-
(S,Sp)-9 (800 mg,
propyloxazolin-2-yl)[2.2]paracyclophane
1.6 mmol) in THF (8 ml), 2 M HCl aq (2.5 mL) was slowly added,
and the resulting solution was stirred at room temperature for
2 h. After completion of the reaction (monitored by TLC), 1 M
NaOH was added dropwise into the solution until the pH reached
Please cite this article in press as: Wang, X.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.05.001

6
X. Wang et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
35.7, 34.1, 33.7, 33.3, 18.9, 18.7, 12.7, 8.2, 7.3. MS (ESI) m/z calcd for
[MÀCl]+ (C32H34N3O): 476.2702, found: 476.2695.
4.3.2. (R)-N-{[Dimethyl(phenyl)silyl](4-tolyl)methyl}-4-toluene-
sulfonamide 14b
Following the general procedure described above, compound
14b was obtained as a white solid: 36.8 mg, 90% yield, 95% ee;
4.2.5. Imidazolium salt (S,SP)-6
(S,SP)-6 was prepared by the same procedure as for (S,SP)-4,
using 2-quinolinecarboxaldehyde instead of 6-methylpyridine-2-
carbaldehyde. The title compound was obtained as a white solid.
[a]
20 = +75 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 11.3 min (S,
minor), tR = 17.3 min (R, major); other spectral and property data
matched those reported in the literature.8
Yield: 75 mg (40%); mp: 200 –202 °C; [
a
]
20 = À213.5 (c 0.1, CH2-
D
Cl2); 1H NMR (300 MHz, CDCl3) d 12.31 (s, 1H), 9.69 (s, 1H),
7.96–7.73 (m, 2H), 7.69–7.54 (m, 1H), 7.59–7.39 (m, 1H), 7.21–
7.08 (m, 1H), 6.97–6.86 (m, 1H), 6.83–6.68 (m, 3H), 6.66–6.57
(m, 1H), 4.72–4.57 (m, 1H), 4.51–4.40 (m, 1H), 4.35–4.15 (m,
1H), 4.08–3.86 (m, 1H), 3.86–3.67 (m, 1H), 3.24–2.94 (m, 4H),
2.91–2.67 (m, 2H), 2.37–2.17 (m, 1H), 2.03–1.85 (m, 1H), 1.89–
1.72 (m, 1H), 1.17 (d, J = 6.6 Hz, 3H), 1.06 (d, J = 6.6 Hz, 3H). 13C
NMR (75 MHz, CDCl3) d 165.4, 142.1, 140.6, 139.5, 137.7, 136.1,
135.1, 134.7, 133.7, 133.3, 131.1, 130.3, 129.7, 129.1, 128.9,
128.3, 126.9, 125.7, 124.3, 119.5, 115.8, 114.8, 71.7, 70.9, 65.3,
35.7, 34.0, 33.6, 33.0, 18.4, 18.6. MS (ESI) m/z calcd for [MÀCl]+
(C33H32N3O): 486.2545, found: 486.2555.
4.3.3. (R)-N-{[Dimethyl(phenyl)silyl](4-methoxyphenyl)-methyl}-
4-toluenesulfonamide 14c
Following the general procedure described above, compound
14c was obtained as a white solid: 39.5 mg, 93% yield, 95% ee;
[a]
20 = +95 (c 0.15, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 12.9 min (S,
minor), tR = 22.4 min (R, major); other spectral and property data
matched those reported in the literature.8
4.3.4. (R)-N-{[Dimethyl(phenyl)silyl](4-trifluoromethylphenyl)
methyl}-4-toluenesulfonamide 14d
4.2.6. Imidazolium salt (SP)-16
(SP)-16 was prepared by the same procedure as for (S,SP)-4,
using enantiopure (SP)-4-amino[2.2]paracyclophane (100 mg,
0.45 mmol) as the starting material. The title compound was
obtained as a white solid. Yield: 97 mg (58%); mp: 228–230 °C;
Following the general procedure described above, compound
14d was obtained as a white solid: 44.4 mg, 96% yield, 96% ee;
[a]
20 = +81 (c 0.15, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 11.3 min (S,
minor), tR = 23.4 min (R, major); other spectral and property data
matched those reported in the literature.8
[a
]
D
20 = À10.3 (c 0.2, CH2Cl2); 1H NMR (500 MHz, CDCl3) d 11.03
(s, 1H), 8.37–8.02 (m, 1H), 7.67–7.62 (m, 1H), 7.57–7.51 (m, 1H),
7.24–7.12 (m, 2H), 6.87–6.78 (m, 1H), 6.62–6.53 (m, 2H), 6.52–
6.47 (m, 1H), 6.45–6.39 (m, 2H), 3.52–3.30 (m, 4H), 3.12–3.04
(m, 2H), 3.02 (s, 3H), 2.96–2.80 (m, 2H). 13C NMR (126 MHz, CDCl3)
d 143.7, 140.5, 138.4, 136.8, 135.3, 134.8, 134.3, 133.5, 133.0,
132.7, 132.2, 130.7, 128.5, 127.4, 125.5, 125.2, 116.6, 115.9,
113.9, 35.0, 34.7, 34.43, 32.4, 19.8. HRMS (ESI) m/z calcd for [MÀ
Cl]+ (C24H23N2): 339.1861, found: 339.1852.
4.3.5. (R)-N-{[Dimethyl(phenyl)silyl](4-fluorophenyl)-methyl}-
4-toluenesulfonamide 14e
Following the general procedure described above, compound
14e was obtained as a white solid: 40.5 mg, 98% yield, 97% ee;
[a]
20 = +84 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 11.4 min (S,
minor), tR = 18.9 min (R, major); other spectral and property data
matched those reported in the literature.8
4.3.
General
procedure
for
the
copper(I)-catalyzed
enantioselective 1,2-silylation of N-tosylaldimines
Imidazolium salt (S,SP)-4 (2.4 mg, 5 Â 10À3 mmol) and Cu2O
(0.35 mg, 2.5 Â 10À3 mmol) were added to 1.0 mL of anhydrous
THF in an oven-dried Schlenk flask under nitrogen. The mixture
was stirred at 60 °C overnight to give a brown solution of the Cu
complex. The solvent was then evaporated under nitrogen at
80 °C, after which NaOMe (8.1 mg, 0.15 mmol, 1.5 equiv) was
added at room temperature, followed by 1.0 mL of anhydrous
toluene. The reaction mixture was stirred at room temperature
for 1 h, and then cooled to 0 °C. At this temperature, PhMe2Si-Bpin
(39 mg, 0.15 mmol, 1.5 equiv) was added via syringe, followed by a
solution of the indicated imine (0.1 mmol, 1.0 equiv) in toluene
(1.0 mL). the reaction was subsequently maintained at 0 °C for
15 min. The reaction mixture was filtered through a small pad of
silica gel and washed with CH2Cl2 (5.0 mL). Evaporation of the sol-
vents under reduced pressure and purification of the residue by
flash column chromatography afforded the corresponding product
14.
4.3.6. (R)-N-{[Dimethyl(phenyl)silyl](4-chlorophenyl)-methyl}-
4-toluenesulfonamide 14f
Following the general procedure described above, compound
14f was obtained as a white solid: 42.2 mg, 98% yield, 95% ee;
[a]
20 = +95.0 (c 0.15, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 11.6 min (S,
minor), tR = 20.5 min (R, major); other spectral and property data
matched those reported in the literature.8
4.3.7. (R)-N-{[Dimethyl(phenyl)silyl](4-bromophenyl)-methyl}-
4-toluenesulfonamide 14g
Following the general procedure described above, compound
14g was obtained as a white solid: 46.0 mg, 97% yield, 94% ee;
[a]
20 = +84 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 11.6 min (S,
minor), tR = 20.5 min (R, major); other spectral and property data
matched those reported in the literature.8
4.3.1. (R)-N-{[Dimethyl(phenyl)silyl](phenyl)methyl}-4-toluene-
sulfonamide 14a
Following the general procedure described above, compound
14a was obtained as a white solid: 37.5 mg, 95% yield, 96% ee;
4.3.8. (R)-N-{[Dimethyl(phenyl)silyl](3-tolyl)methyl}-4-toluene-
sulfonamide 14h
[a]
20 = +65 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IA column: k = 220 nm, solvent:
hexane/i-PrOH (15: 1), flow rate = 1.0 mL/min; tR = 11.0 min (S,
minor), tR = 13.7 min (R, major); other spectral and property data
matched those reported in the literature.8
Following the general procedure described above, compound
14h was obtained as a white solid: 39.6 mg, 97% yield, 86% ee;
[a]
20 = +28 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
Please cite this article in press as: Wang, X.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.05.001

X. Wang et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
7
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 11.2 min (S,
minor), tR = 18.1 min (R, major); other spectral and property data
matched those reported in the literature.8
4.3.15. (R)-N-{[Dimethyl(phenyl)silyl](naphthalen-2-yl)-methyl}-
4-toluenesulfonamide 14o
Following the general procedure described above, compound
14o was obtained as a white solid: 42.5 mg, 95% yield, 94% ee;
[a]
20 = +175 (c 0.2, CH2Cl2); The enantiomeric excess was deter-
D
4.3.9. (R)-N-{[Dimethyl(phenyl)silyl](3-methoxyphenyl)-methyl}-
4-toluenesulfonamide 14i
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 12.2 min (S,
minor), tR = 19.0 min (R, major); other spectral and property data
matched those reported in the literature.8
Following the general procedure described above, compound
14i was obtained as a white solid: 40.8 mg, 96% yield, 87% ee;
[a]
20 = +88 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 12.8 min (S,
minor), tR = 17.0 min (R, major); other spectral and property data
matched those reported in the literature.8
4.3.16. (R)-N-{[Dimethyl(phenyl)silyl](thiophen-2-yl)-methyl}-
4-toluenesulfonamide 14p
Following the general procedure described above, compound
14p was obtained as a white solid: 36.0 mg, 90% yield, 86% ee;
[a]
20 = +66 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
4.3.10. (R)-N-{[Dimethyl(phenyl)silyl](3-trifluoromethyl-phenyl)
methyl}-4-toluenesulfonamide 14j
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4:1), flow rate = 0.5 mL/min; tR = 12.0 min (S,
minor), tR = 20.6 min (R, major); other spectral and property data
matched those reported in the literature.8
Following the general procedure described above, compound
14j was obtained as a white solid: 45.0 mg, 97% yield, 91% ee;
[a]
20 = +62 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 10.5 min (S,
minor), tR = 13.2 min (R, major); other spectral and property data
matched those reported in the literature.8
4.3.17. (R)-N-{[Dimethyl(phenyl)silyl](cyclohexyl)methyl}-4-
toluenesulfonamide 14q
Following the general procedure described above, compound
14q was obtained as a colorless oil: 36.0 mg, 90% yield, 94% ee;
[
a
]
20 = À5 (c 0.2, CH2Cl2); The enantiomeric excess was determined
D
4.3.11. (R)-N-{[Dimethyl(phenyl)silyl](2-tolyl)methyl}-4-toluene-
sulfonamide 14k
by HPLC with a Chiralpak IB column: k = 220 nm, solvent: hexane/
i-PrOH (25:1), flow rate = 0.5 mL/min; tR = 18.4 min (S, minor),
tR = 20.2 min (R, major); other spectral and property data matched
those reported in the literature.8
Following the general procedure described above, compound
14k was obtained as a white solid: 39.5 mg, 96% yield, 95% ee;
[a]
20 = +68 (c 0.15, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 10.0 min (S,
minor), tR = 13.5 min (R, major); other spectral and property data
matched those reported in the literature.8
Acknowledgments
Financial support from the National Natural Science Foundation
of China (Grant Nos. 21372144, 81473085) and Department of
Science and Technology of Shandong Province is gratefully
acknowledged.
4.3.12. (R)-N-{[Dimethyl(phenyl)silyl](2-methoxyphenyl)-methyl}-
4-toluenesulfonamide 14l
References
Following the general procedure described above, compound
14l was obtained as a white solid: 40.0 mg, 94% yield, 95% ee;
1. For reviews on the use of organosilanes in organic synthesis, see: (a) Chan, T.;
Wang, D. Chem. Rev. 1992, 92, 995; (b) Jones, G. R.; Landais, Y. Tetrahedron 1996,
52, 7599; (c) Fleming, I.; Barbero, A.; Walter, D. Chem. Rev. 1997, 97, 2063; (d)
Suginome, M.; Ito, Y. Chem. Rev. 2000, 100, 3221; (e) Hartmann, E.; Vyas, D. J.;
Oestreich, M. Chem. Commun. 2011, 7917.
2. For a review on silylboron reagents, see: (a) Ohmura, T.; Suginome, M. Bull.
Chem. Soc. Jpn. 2009, 82, 29; (b) Oestreich, M.; Hartmann, E.; Mewald, M. Chem.
Rev. 2013, 113, 402.
[a]
20 = +68 (c 0.2, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4: 1), flow rate = 0.5 mL/min; tR = 11.4 min (S,
minor), tR = 14.3 min (R, major); other spectral and property data
matched those reported in the literature.8
3. (a) Walter, C.; Auer, G.; Oestreich, M. Angew. Chem., Int. Ed. 2006, 45, 5675; (b)
Walter, C.; Oestreich, M. Angew. Chem., Int. Ed. 2008, 47, 3818; (c) Lee, K.-S.;
Hoveyda, A. H. J. Am. Chem. Soc. 2010, 132, 2898; (d) Ibrahem, I.; Santoro, S.;
Himo, F.; Córdova, A. Adv. Synth. Catal. 2011, 353, 245; (e) Lee, K.-S.; Wu, H.;
Haeffner, F.; Hoveyda, A. H. Organometallics 2012, 31, 7823; (f) Pace, V.; Rae, J.
P.; Harb, H. Y.; Procter, D. J. Chem. Commun. 2013, 5150; (g) Pace, V.; Rae, J. P.;
Procter, D. J. Org. Lett. 2014, 16, 476; (h) O’Brien, J. M.; Hoveyda, A. H. J. Am.
Chem. Soc. 2011, 133, 7712.
4. For reviews, see: (a) Sieburth, S. M.; Chen, C. A. Eur. J. Org. Chem. 2006, 311; (b)
Sieburth, S. M.; Nittoli, T.; Mutahi, A. M.; Guo, L. Angew. Chem., Int. Ed. 1998, 37,
812; (c) Mutahi, M. W.; Nittoli, T.; Guo, L.; Sieburth, S. M. J. Am. Chem. Soc. 2002,
124, 7363; (d) Mortensen, M.; Husmann, R.; Veri, E.; Bolm, C. Chem. Soc. Rev.
2009, 1002, 38.
4.3.13. (R)-N-{[Dimethyl(phenyl)silyl](2-trifluoromethylphenyl)
methyl}-4-toluenesulfonamide 14m
Following
compound 14m was obtained as a white solid: 44.5 mg, 96%
yield, 92% ee; [
20 = +106 (c 0.2, CH2Cl2); The enantiomeric
the
general
procedure
described
above,
a]
D
excess was determined by HPLC with a Chiralpak IB column:
k = 220 nm, solvent: hexane/i-PrOH (4: 1), flow rate = 0.5 mL/
min; tR = 12.1 min (S, minor), tR = 20.9 min (R, major); other
spectral and property data matched those reported in the
literature.8
5. Vyas, D. J.; Fröhlich, R.; Oestreich, M. Org. Lett. 2011, 13, 2094.
6. Hensel, A.; Nagura, K.; Delvos, L. B.; Oestreich, M. Angew. Chem., Int. Ed. 2014,
53, 4964.
7. Zhao, C.; Jiang, C.; Wang, J.; Wu, C.; Zhang, Q.; He, W.; Asian, J. Org. Chem. 2014,
3, 851.
4.3.14. (R)-N-{[Dimethyl(phenyl)silyl](naphthalen-1-yl)-methyl}-
4-toluenesulfonamide 14n
8. Chen, Z.; Huo, Y.; An, P.; Wang, X.; Song, C.; Ma, Y. Org. Chem. Front. 2016, 3,
1725.
Following the general procedure described above, compound
14n was obtained as a white solid: 42.0 mg, 94% yield, 94% ee;
9. Mita, T.; Sugawara, M.; Saito, K.; Sato, Y. Org. Lett. 2014, 16, 3028.
10. (a) Pye, P. J.; Rossen, K.; Reamer, R. A.; Tsou, N. N.; Volante, R. P.; Reider, P. J. J.
Am. Chem. Soc. 1997, 119, 6207; (b) Hermanns, N.; Dahmen, S.; Bolm, C.; Bräse,
S. Angew. Chem., Int. Ed. 2002, 41, 3692; (c) Gibson, S. E.; Knight, J. D. Org.
Biomol. Chem. 2003, 1, 1256; (d) Wu, X.; Zhang, T.; Hou, X. Tetrahedron:
Asymmetry 2004, 15, 2357; (e) Bräse, S.; Höfener, S. Angew. Chem., Int. Ed. 2005,
44, 7879; (f) Whelligan, D. K.; Bolm, C. J. Org. Chem. 2006, 71, 4609; (g) Fürstner,
[a]
20 = +17 (c 0.1, CH2Cl2); The enantiomeric excess was deter-
D
mined by HPLC with a Chiralpak IB column: k = 220 nm, solvent:
hexane/i-PrOH (4:1), flow rate = 0.5 mL/min; tR = 11.5 min (S,
minor), tR = 18.4 min (R, major); other spectral and property data
matched those reported in the literature.8
Please cite this article in press as: Wang, X.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.05.001

8
X. Wang et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
A.; Alcarazo, M.; Krause, H.; Lehmann, C. W. J. Am. Chem. Soc. 2007, 129, 12676; 13. Niu, Z.; Chen, J.; Chen, Z.; Ma, M.; Song, C.; Ma, Y. J. Org. Chem. 2015, 80, 602–
(h) Paradies, J. Synthesis 2011, 23, 3749; (i) Kitagaki, S.; Ueda, T.; Mukai, C.
Chem. Commun. 2013, 4030; (j) Jiang, B.; Lei, Y.; Zhao, X. J. Org. Chem. 2008, 73,
7833; (k) Rozenberg, V.; Sergeeva, E.; Hopf, H. In Modern Cyclophane Chemistry;
Wily-VCH: Weinheim, Germany, 2004; p 435; (l) Zhao, L.; Ma, Y.; Duan, W.; He,
F.; Chen, J.; Song, C. Org. Lett. 2012, 14, 5780.
608.
14. Bolm, C.; Focken, T.; Raabe, G. Tetrahedron: Asymmetry 2003, 14, 1733.
15. (a) Duan, W.; Ma, Y.; Xia, H.; Liu, X.; Ma, Q.; Sun, J. J. Org. Chem. 2008, 73, 4330;
(b) Qu, B.; Ma, Y.; Ma, Q.; Liu, X.; He, F.; Song, C. J. Org. Chem. 2009, 74, 6867.
16. Opalka, S. M.; Park, J. K.; Longstreet, A. R.; McQuade, D. T. Org. Lett. 2013, 15,
996.
11. Bolm, C.; Kesselgruber, M.; Raabe, G. Organometallics 2002, 21, 707.
12. Hong, B.; Ma, Y.; Zhao, L.; Duan, W.; He, F. Tetrahedron: Asymmetry 2011, 22,
1055.
17. Pelter, A.; Crump, R. Tetrahedron: Asymmetry 1997, 8, 3873.
Please cite this article in press as: Wang, X.; et al. Tetrahedron: Asymmetry (2017), http://dx.doi.org/10.1016/j.tetasy.2017.05.001