P-stereogenic diphosphacrowns: Facile incorporation of aromatic rings

Article abstract of DOI:10.3987/COM-15-13324

P-Stereogenic diphosphacrowns containing naphthalene and pyridine rings were synthesized. Facile incorporation of aromatic rings, and chains of different lengths, into the diphosphacrown skeleton was achieved by altering the electrophile in our previously

Full text of DOI:10.3987/COM-15-13324

HETEROCYCLES, Vol. 91, No. 12, 2015, pp. -. © 2015 The Japan Institute of Heterocyclic Chemistry
Received, 14th September, 2015, Accepted, 12th November, 2015, Published online, 27th November, 2015
DOI: 10.3987/COM-15-13324
P-STEREOGENIC DIPHOSPHACROWNS: FACILE INCORPORATION
OF AROMATIC RINGS
a
a
b,
Ryosuke Kato, Hiroyuki Watanabe, Yasuhiro Morisaki, * and Yoshiki
a,
Chujo *
a
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto
University,
Nishikyo-ku,
Kyoto
615-8510,
Japan.
E-mail:
chujo@chujo.synchem.kyoto-u.ac.jp
b
Department of Applied Chemistry for Environment, School of Science and
Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337,
Japan. E-mail: ymo@kwansei.ac.jp
Abstract – P-Stereogenic diphosphacrowns containing naphthalene and pyridine
rings were synthesized. Facile incorporation of aromatic rings, and chains of
different lengths, into the diphosphacrown skeleton was achieved by altering the
electrophile in our previously reported synthetic method. Pyridine-containing
diphosphacrown exhibited chiral recognition ability for chiral ammonium salts
and carboxylic acids. This is the first example of chiral recognition using
1
31
P-stereogenic diphosphacrowns. H and P NMR spectra indicated that the
nitrogen, oxygen, and chiral phosphorus atoms contributed to the chiral
recognition cooperatively.
INTRODUCTION
Phosphines are configurationally stable because of the high s character of the phosphorus atoms. The
resulting high energy barrier for phosphines inversion allows the phosphorus atoms to become chiral
1
,2
centers.
Various P-stereogenic phosphines have been prepared till date, with most of them used as
2
asymmetric ligands in transition metal-catalyzed enantioselective reactions.
3
Our research has focused on P-stereogenic phosphines, employing them as chiral element-blocks in the
4
synthesis of P-stereogenic well-defined oligomers, polymers, and cyclic compounds. Among these
cyclic compounds, P-stereogenic phosphacrowns were the first example of crown ethers with chiral
5
heteroatoms that interacted directly with guest molecules. Since the first report of crown ether
6
(
dibenzo-18-crown-6) in 1967 by Pedersen, various types and derivatives of crown ethers have been

7
8
9
reported. Optically active crown ethers containing stereogenic carbon atoms or axially chiral units in
the ring skeleton have been shown to exhibit chiral guest-recognition abilities.
Recently, a practical synthetic route to P-stereogenic benzodiphosphacrowns was developed using
P-stereogenic secondary bisphosphine precursors. In this study, we expand the substrate scope in the
synthesis of P-stereogenic diphosphacrowns to incorporate different aromatic units (naphthalene and
pyridine) and ring sizes. Their preparation and characterization, including chiral recognition studies, are
reported.
RESULTS AND DISCUSSION
The synthetic route to naphthalene-containing P-stereogenic diphosphacrowns is shown in Scheme 1.
P-Stereogenic secondary bisphosphine borane complex (S,S)-1–BH was prepared as previously
3
1
0
described.
Bisphosphine (S,S)-1–BH was readily lithiated with n-BuLi, and successive treatment with
3
electrophiles 2a-c under diluted conditions afforded the corresponding P-stereogenic
1
1
(
R,R)-naphthodiphosphacrowns (R,R)-3a-c–BH , after purification by SiO column chromatography.
3 2
1
13
31
The products were characterized by H, C, and P NMR spectroscopy and high-resolution mass
spectrometry (HRMS). The absolute configuration of the phosphine was unaltered during this
procedure; however, a change in the priority of the P-substituents resulted in different R/S labels for the
optical isomers.
BH3
P
BH3
P
n-BuLi
H
Li
P
H
P
Li
THF
BH3
BH3
(
S,S)-1–BH3
O
O
O
OTs
OTs
n
O
O
O
O
O
O
O
2
a-c
O
O
O
O
a, n = 1
b, n = 2
c, n = 3
P
P
P
P
P
P
(
R,R)-3b–BH3
(R,R)-3c–BH3
(
R,R)-3a–BH3
6
9%
47%
2
5%
P = P
BH3
Scheme 1

A suitable single crystal of (R,R)-3c–BH was obtained, and its structure was confirmed by X-ray
3
crystallography. The ORTEP drawing is shown in Figure 1; crystallographic data are listed in Tables S1
1
2
and S2 in the Supporting Information
X-Ray crystallographic analysis revealed that the crystal
structure of (R,R)-3c–BH was similar to that of (R,R)-benzo-24-diphosphacrown-8 previously. We
3
confirmed that the 1:1 reaction stoichiometry in Scheme 1 resulted in preferential construction of
2
4-diphosphacrown-8 over the 48- or 72-membered rings. The 24-diphosphacrown-8 ring possessed
two P-stereogenic centers, both with R absolute configuration, and two tert-butyl substituents on the
phosphorus atoms located at diagonal quadrants.
Figure 1. ORTEP drawing of (R,R)-3c–BH . Thermal ellipsoids are drawn at the 30% probability level.
3
Hydrogen atoms are omitted for clarity.
Pyridine-containing
,6-bis(hydroxymethyl)pyridine 6, as shown in Scheme 2. P-Stereogenic diphosphacrown containing a
pyridine ring was synthesized using the same procedure as that used for the naphthodiphosphacrowns.
Accordingly, treatment of (S,S)-1–BH with n-BuLi, followed by the addition of electrophile 10, afforded
R,R)-pyrido-21-diphosphacrown-7 (R,R)-11–BH in 43% isolated yield (Scheme 3). The structure of
was confirmed by H, C, and P NMR, and HRMS spectra.
electrophile
10
was
synthesized
from
commercially
available
2
3
(
(
3
1
13
31
R,R)-11–BH
3

N
N
TsCl, NaOH aq
THF
TsO
OTs
HO
OH
5
3%
4
5
1
2
) NaH
) 5
,
PPTS
O
HO
OH
HO
OTHP
O
O
CH Cl
53%
THF
2
2
7
6
PPTS =pyridinium p-toluenesulfonate
THP = tetrahydropyranyl
THPO
N
OTHP
HO
N
OH
EtOH, PPTS
4%
O
O
O
O
2
2
2
2
6
from 7
8
9
TsO
N
OTs
TsCl, NaOH aq
THF
O
O
2
2
7
8%
1
0
Scheme 2
BH3
N
O
O
n-BuLi
THF
10
Li
P
(
S,S)-1–BH3
P
Li
THF
O
O
4
3%
BH3
P
P
P = P
BH3
(R,R)-11–BH3
Scheme 3
Boranes, which protect the phosphorus lone pairs, can be readily removed using strong acids (e.g.
1
3
trifluoromethanesulfonic acid or tetrafluoroboric acid) or organic bases (e.g. morpholine or
1
4
1
,4-diazabicyclo[2.2.2]octane) without causing pyramidal inversion. In this study, borane was
removed from (R,R)-11–BH using trifluoromethanesulfonic acid and aqueous potassium hydroxide, as
shown in Scheme 4. Extraction with diethyl ether and purification by Al column chromatography
3
2
O
3

afforded free phosphine (R,R)-11 in 72% yield as an air-sensitive colorless liquid, which was
1
31
characterized by H NMR, P NMR, and HRMS spectra.
N
N
O
O
O
O
1
) CF SO H, toluene
3
3
2) KOH aq
O
BH3 BH O
3
O
O
7
2%
P
P
P
P
(
R,R)-11−BH3
(R,R)-11
Scheme 4
H
H
NH3+ ClO4–
NH3+ ClO4–
Me
Me
(S)-12
(
R)-12
Figure 2. Structures of guest molecules
Figure 3. The CH
3
proton peaks around 1.8 ppm in (R)-12 with 1 eq .of (R,R)-11, (S)-12 with 1 eq. of
/CD CN (1:1, v/v)
(R,R)-11, and (S)-12 in CDCl
3
3

1
Figure 4. H NMR spectra of D-13, (R,R)-11−D-13, (R,R)-11, (R,R)-11−L-13, and L-13
1
We investigated the chiral recognition ability of (R,R)-11 by monitoring changes in H NMR spectra after
treatment with perchlorate salts of enantiomers of (1-naphthyl)ethylammonium ((R)/(S)-12) and
3
-phenyllactic acid (D/L-13), as shown in Figure 2. When an equimolar amount of guest 12 was added
to (R,R)-11 in CDCl /CD CN solution (v/v = 1/1), peak shifts (Δδ) were observed for the O-CH (Figure
S17 in the Supporting Information), and pyridyl protons in (R,R)-11 as well as the CH and CH protons in
2. As shown in Figure 3, the CH proton peak in 12, usually around 1.8 ppm, was distinctly shifted
3
3
2
3
1
3
downfield by host-guest interactions, giving Δδ values of 0.0172 ppm for (R,R)-11–(R)-12 and 0.0145
ppm for (R,R)-11–(S)-12. Despite the small differences in Δδ between (R,R)-11–(R)-12 and (R,R)-11–
(S)-12, this was clear evidence of (R,R)-11 exhibiting chiral recognition; the complexation constants were
–
1
estimated to be approximately 64.5 and 46.9 M , respectively. In addition, (R,R)-11 recognized
1
enantiomers D-13 and L-13, as confirmed by H NMR spectra (Figure 4). In both cases, the (R,R)-11
2
methylene (O–CH ) peaks at around 3.4-3.8 and 4.6 ppm shifted noticeably, suggesting hydrogen
bonding between ether oxygen atoms and acidic protons, in addition to those between pyridyl nitrogen
and acidic protons. To elucidate the interaction between the phosphorus atoms and the guest molecule,
3
1
P NMR spectra of (R,R)-11 and (R,R)-11−D-13 were obtained (Figure S20 in the Supporting
3
1
Information). The P NMR signal became broader and appeared at a slightly lower magnetic field in
R,R)-11−D-13 than in (R,R)-11. Adding an excess of trifluoroacetic acid (TFA), a strong acid, caused a
(

sharp peak to appear at +30 ppm, indicating protonated phosphorus atoms. These results implied that
phosphorus atoms constructed a chiral environment rather than had an affinity for hydrogen atoms in the
guest molecule.
CONCLUSION
We have demonstrated the facile incorporation of aromatic rings into P-stereogenic diphosphacrowns,
affording naphthalene- and pyridine-containing diphosphacrowns. Various naphthalene-containing
P-stereogenic diphosphacrown ring sizes were obtained by altering the electrophile used for cyclization.
A preliminary chiral recognition experiment, involving mixing of chiral ammonium salts and carboxylic
acids with the pyridine-containing diphosphacrown, revealed different chemical shifts for each guest
enantiomer. It was confirmed that, in addition to nitrogen and oxygen, phosphorus atoms contributed to
the capture of guest molecules. Studies on the synthesis of optically active diphosphacrowns containing
a variety of heteroaromatic rings, and their applications as NMR chiral shift reagents, are underway.
EXPERIMENTAL
1
13
General. H (400 MHz) and C (100 MHz) NMR spectra were recorded on a 400 MHz spectrometer,
3
1
and samples were analyzed in CDCl or CD Cl using Me Si as an internal standard.
P (161.5 MHz)
3
2
2
4
NMR spectra were also recorded on a 400 MHz spectrometer, and samples were analyzed in CDCl using
3
H PO as an external standard. High-resolution mass spectra (HRMS) were obtained by electron spray
3
4
ionization (ESI) or direct analysis in real time (DART) technique using an Orbitrap mass spectrometer
EXACTIVE, Thermo Fisher Scientific). Optical rotation was measured using CHCl as a solvent.
(
3
Analytical thin-layer chromatography was performed with MERCK SiO2 plates.
chromatography was performed with Wakogel C-300 SiO or MERCK Al O 90 active basic.
Column
2
2
3
Materials. THF and Et O were purchased and purified by the GlassContour solvent purification
2
1
5
10
system.
Compound (S,S)-1–BH3 was synthesized as described in the literatures. The other materials
were purchased and used without further purification.
Synthesis of (R,R)-3a–BH . A THF solution (60 mL) of (S,S)-1–BH (233.9 mg, 1.0 mmol) was cooled
3
3
to −78 °C. n-BuLi (1.6 M in n-hexane, 1.40 mL, 2.2 mmol) was added with a syringe. After stirring
for 1 h, a THF solution (20 mL) of electrophile 2a (556.6 mg, 1.0 mmol) was added with a syringe. The
reaction mixture was allowed to warm to room temperature. After stirring for 48 h, the reaction was
quenched with 2 N HCl aq (50 mL). The organic layer was extracted with AcOEt (50 mL × 3). The
combined organic layers were washed with saturated NaHCO aq, brine, and dried over MgSO . After
3
4
filtration, the solvent was removed in vacuo. The residue was subjected to column chromatography on

SiO with AcOEt and hexane (v/v = 1/3) as an eluent. Removal of the solvent gave (R,R)-3a–BH3
2
2
D
3
(
109.9 mg, 0.25 mmol, 25%) as a colorless solid. R = 0.75 (hexane/AcOEt, v/v = 1:1); [α]
+48.6 (c
f
1
0
1
.5, CHCl ); H NMR (CDCl , 400 MHz) δ 0.38 (br q, J = 80.8 Hz, 6H, BH ), 1.17 (d, J = 13.5 Hz,
3
3
H-B
3
8H, C(CH ) ), 1.96-2.04 (m, 4H, PCH ), 2.20-2.30 (m, 2H, PCH ), 2.47-2.56 (m, 2H, PCH ), 4.36-4.50
3
3
2
2
2
1
3
(
m, 4H, OCH ), 7.03 (s, 2H, C H ), 7.31-7.33 (m, 2H, C H ), 7.64-7.66 (m, 2H, C H ) ppm; C NMR
2 10 6 10 6 10 6
(
CDCl , 100 MHz) δ 15.3 (d, J = 30.0 Hz), 21.3 (d, J = 30.0 Hz), 25.3, 28.4 (d, J = 34.4 Hz), 63.1,
3 C-P C-P C-P
3
1
1
1
06.6 (d, J = 9.7 Hz), 124.3, 126.2 (d, J = 5.8 Hz), 129.0, 147.5 ppm; P{ H}NMR (CDCl , 161.5 MHz)
3
+
δ +34.6 (br) ppm. HRMS (ESI) calcd for [C H B O P +NH ] : 464.3193, found 464.3188.
2
4
42
2
2
2
4
Synthesis of (R,R)-3b–BH and (R,R)-3c–BH . Synthetic procedures are same as that of (R,R)-3a–
3
3
BH ; compound data are as follows.
3
2
D
3
1
(
R,R)-3b–BH : R = 0.65 (AcOEt and hexane, v/v = 1/1). [α]
+44.8 (c 0.5, CHCl ); H NMR (CDCl ,
3 3
3
f
4
00 MHz) δ 0.36 (br q, JH-B = 111.2 Hz, 6H, BH ), 1.15 (d, J = 13.7 Hz, 18H, C(CH ) ), 1.84-2.05 (m, 8H,
3 3 3
PCH ), 3.81-3.97 (m, 8H, OCH ), 4.22-4.26 (m, 4H, OCH ), 7.13 (s, 2H, C H ), 7.30-7.35 (m, 2H, C H ),
2
2
2
10
6
10
6
1
3
7
.64-7.67 (m, 2H, C H ) ppm; C NMR (CDCl , 100 MHz) δ15.2 (d, J = 30.6 Hz,), 21.3 (d, J = 30.2
1
0
6
3
C-P
C-P
3
1
1
Hz), 25.2, 28.4 (d, JC-P = 34.1 Hz), 66.4, 68.1, 69.6, 108.8, 124.4, 126.3, 129.3, 149.1 ppm; P{ H}NMR
+
(
CDCl , 161.9 MHz) δ +32.3 (br d, JP-B = 35.0 Hz) ppm. HRMS (ESI) calcd for [C H B O P +NH ] :
3
28 50
2
4
2
4
5
52.3709, found 552.3699.
2
D
3
1
(
R,R)-3c–BH : R = 0.20 (hexane/AcOEt, v/v = 1:1); [α]
+1.2 (c 0.5, CHCl ); H NMR (CDCl , 400
3 3
3
f
MHz) δ 0.34 (br q, JH-B = 117.4 Hz, 6H, BH ), 1.14 (d, J = 13.8 Hz, 18H, C(CH ) ), 1.84-2.02 (m, 8H,
3
3 3
PCH ), 3.60-3.87 (m, 12H, OCH ), 3.95-3.97 (m, 4H, OCH ), 4.25-4.27 (m, 4H, OCH ), 7.12 (s, 2H,
2
2
2
2
1
3
C H ), 7.31-7.33 (m, 2H, C H ), 7.64-7.67 (m, 2H, C H ) ppm; C NMR (CDCl , 100 MHz) δ 15.3 (d,
1
0
6
10
6
10
6
3
J = 30.8 Hz), 21.4 (d, J = 30.4 Hz), 25.3, 28.5 (d, J = 33.7 Hz), 66.3, 69.0, 69.6, 70.4, 70.9, 108.4,
C-P
C-P
C-P
3
1
1
1
24.2, 126.3, 129.4, 149.1 ppm; P{ H}NMR (CDCl , 161.5 MHz) δ +32.3 (br) ppm. HRMS (ESI) calcd
3
+
for [C H B O P +NH ] : 640.4233, found 640.4222.
3
2
58
2
6
2
4
Synthesis of 10. A mixture of an aqueous solution (10 mL) of NaOH (1.7 g, 42 mmol) and a THF
solution (20 mL) of 9 (1.65 g, 5.23 mmol) was cooled to 0 °C. To this solution was added a THF
solution (20 mL) of TsCl (5.0 g, 26 mmol) in one portion. The solution was allowed to warm to room
temperature and stirred overnight. The reaction was quenched with H O, and the organic layer was
2
extracted with CH Cl (50 mL × 3). The combined organic layers were washed with H O three times
2
2
2
and brine, and then dried over Na SO . After filtration, the solvent was removed in vacuo. The residue
2
4
was subjected to column chromatography on SiO with AcOEt and hexane (v/v = 4/1) as an eluent. The
2
solvent was evaporated to obtain 10 (2.55 g, 4.08 mmol, 78%) as colorless oil. R = 0.10 (hexane/AcOEt,
f
1
v/v = 1:4); H NMR (CDCl , 400 MHz) δ 2.43 (s, 3H, Ar-CH ), 3.6-3.8 (m, 12H, -O-CH ), 4.18 (t, J = 4.9
3
3
2

Hz, 4H, Ar-SO -CH ), 4.63 (s, 4H, Ar-CH -O), 7.3-7.4 (m, 6H, Ar), 7.72 (t, J = 7.6 Hz, 1H, Ar), 7.79 (d, J
3
2
2
1
3
=
8.6 Hz, 4H, Ar) ppm; C NMR (CDCl , 100 MHz) δ 21.6, 68.8, 69.2, 70.2, 70.7, 74.1, 120.0, 128.0,
3
+
1
6
29.8, 133.2, 137.4, 144.7, 157.8 ppm; HRMS (ESI) calcd for [C H NO S +H] 624.1932, found
2
9
37
10 2
24.1917.
Synthesis of (R,R)-11–BH
to −78 °C under Ar atmosphere. To this solution, n-BuLi (1.60 M in cyclohexane and n-hexane, 0.6 mL,
.0 mmol) was added by a syringe. After the solution was stirred for 1 h, a THF solution (20 mL) of 10
0.25 g, 0.39 mmol) was added over a period of 5 min by a syringe. After 1 h, the mixture was allowed
3
3
. A THF solution (30 mL) of (S,S)-1–BH (0.093 g, 0.39 mmol) was cooled
1
(
to warm to room temperature and stirred for 72 h. The reaction was quenched with H O (30 mL), and
2
the organic layer was extracted with AcOEt (50 mL × 3). The combined organic layers were washed
with brine, and then dried over MgSO . After filtration, the solvent was removed in vacuo. The
4
residue was subjected to column chromatography on SiO with AcOEt and hexane (v/v = 4/1) as an eluent.
2
The solvent was evaporated to obtain (R,R)-11–BH (0.085 g, 0.17 mmol, 43%) as colorless oil. R =
3
f
2
3
1
0
.20 (hexane/AcOEt, v/v = 1:4); [α] +4.3 (c 0.25, CHCl ); H NMR (CD Cl , 400 MHz) δ −0.5-0.8 (br
D 3 2 2
q, JH-B = 75 Hz, 6H, BH ), 1.0 (br, 18H, t-Bu), 1.5-2.0 (m, 8H, P-CH ), 3.2-3.8 (m, 12H, O-CH ), 4.4-4.6
3
2
2
1
3
(
q, J = 9.5 Hz, 4H, Ar-CH -O), 7.18 (d, J = 7.4 Hz, 2H, Ar), 7.59 (t, J = 7.4 Hz, 1H, Ar) ppm; C NMR
2
(
CDCl , 100 MHz) δ 15.3 (d, JC-P = 30 Hz), 20.4 (d, JC-P = 30 Hz), 25.3, 28.6 (d, JC-P = 33 Hz), 66.1, 70.0,
3
3
1
1
7
0.6, 74.4, 121.0, 136.9, 158.0 ppm; P{ H} NMR (CDCl , 161.5 MHz) δ +33.0 (br d, J = 47 Hz)
3 P-B
+
ppm; HRMS (ESI) calcd for [C H B NO P +H] 514.3552, found 514.3549.
2
5
51
2
4 2
Removal of BH from (R,R)-11–BH . To a solution of (R,R)-11–BH (0.016 g, 0.032 mmol) in
3
3
3
degassed dry toluene (0.5 mL) was added trifluoromethanesulfonic acid (0.5 mL, 6 mmol) by a syringe
under Ar atmosphere. After stirring for 2 h at room temperature, toluene was removed in vacuo. The
residue was dissolved in degassed EtOH (6 mL) and added to degassed KOH aq (0.6482 g, 12 mmol
KOH in 10 mL H O). After the reaction for 5 h at 50 °C under Ar atmosphere, the solution was allowed
2
to cool to room temperature. Extraction with degassed dry Et O was carried out, and the organic layer
2
was dried over MgSO . After filtration, the solvent was removed in vacuo. The residue was purified
4
by column chromatography on Al O (active basic) to obtain (R,R)-11 (0.011 g, 0.023 mmol, 72%) as
2
3
1
colorless oil. H NMR (CDCl , 400 MHz) δ 1.0 (br, 18H, t-Bu), 1.4-1.8 (m, 8H , P-CH ), 3.4-3.8 (m,
3
2
1
2H, O-CH ), 4.6-4.8 (q, J = 13 Hz, 4H, Ar-CH -O), 7.31 (d, J = 7.7 Hz, 2H, Ar), 7.67 (t, J = 7.7 Hz, 1H,
2 2
3
1
1
+
Ar) ppm; P{ H} NMR (CDCl , 161.5 MHz) δ −0.6 ppm; HRMS (DART) calcd for [C H NO P +H]
3
25 45
4 2
4
86.2897, found 486.2893.
Host-guest reaction. Sample preparation was performed under Ar atmosphere. A CDCl solution of
3
(
R,R)-11 was prepared. The concentration of this solution was determined as 27.16 mM by the
1
quantitative H NMR method using dimethyl sulfone as an internal standard. A CD CN solution of
3

(
R)-12 or (S)-12 (9.05 mM) was prepared. CDCl and CD CN solutions were mixed and stirred for 1 h
3 3
1
at room temperature; then, H NMR measurement was carried out.
A CDCl solution of (R,R)-11 was prepared. The concentration of this solution was determined as 4.44
3
1
mM by the quantitative H NMR method using dimethyl sulfone as an internal standard. A CD CN
3
solution of D-13 or L-13 (4.44 mM) was prepared. CDCl and CD CN solutions were mixed, and then
3
3
1
31
H and P NMR measurements were carried out. After NMR measurement, excess CF CO H (ca. 10 μl)
3
2
1
31
was added to the solution, and the solution was shaken for a minute; then, H and P NMR measurements
were carried out.
ACKNOWLEDGEMENTS
This work was supported by Grant-in-Aid for the Scientific Research on Innovative Areas of “New
Polymeric Materials Based on Element-Blocks” (No. 15H00733) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. R.K. appreciates Research Fellowships (No. 14J02401) from
the Japan Society for the Promotion of Science for Young Scientists.
REFERENCES AND NOTES
1
.
(a) R. D. Baechler and K. Mislow, J. Am. Chem. Soc., 1970, 92, 3090; (b) A. Rauk, J. D. Andose, W.
G. Frick, R. Tang, and K. Milsow, J. Am. Chem. Soc., 1971, 93, 6507.
2
.
(a) J. Meisenheimer and L. Lichtenstadt, Chem. Ber., 1911, 44, 356; (b) A. Grabulosa, P-Stereogenic
Ligands in Enantioselective Catalysis, RSC Publishing, Cambridge, 2011. And also, for example,
see: W. S. Knowles, Angew. Chem. Int. Ed., 2002, 41, 1998; (c) W. Tang and X. Zhang, Chem. Rev.,
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