Synthesis of a chiral N-heterocyclic carbene bearing a m-terphenyl-based phosphate moiety as an anionic N-substituent and its application to copper-catalyzed enantioselective boron conjugate additions

Article abstract of DOI:10.1016/j.tetasy.2013.05.013

A chiral N-heterocyclic carbene (NHC) ligand 1a bearing a m-terphenyl-based phosphate moiety as an anionic N-substituent has been developed. A rhodium complex [Rh(1a)(cod)]₂ was synthesized and its structure was characterized by NMR and ESI-MS spectroscopy. This ligand gave high enantioselectivities in copper-catalyzed enantioselective boron conjugate additions to an α,β-unsaturated ester to give a chiral β-boryl ester.

Full text of DOI:10.1016/j.tetasy.2013.05.013

Tetrahedron: Asymmetry 24 (2013) 729–735
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Synthesis of a chiral N-heterocyclic carbene bearing a m-terphenyl-
based phosphate moiety as an anionic N-substituent and its
application to copper-catalyzed enantioselective boron conjugate
additions
⇑
Tomohiro Iwai, Yuki Akiyama, Masaya Sawamura
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 April 2013
Accepted 7 May 2013
A chiral N-heterocyclic carbene (NHC) ligand 1a bearing a m-terphenyl-based phosphate moiety as an
anionic N-substituent has been developed. A rhodium complex [Rh(1a)(cod)]2 was synthesized and its
structure was characterized by NMR and ESI-MS spectroscopy. This ligand gave high enantioselectivities
in copper-catalyzed enantioselective boron conjugate additions to an a,b-unsaturated ester to give a chi-
ral b-boryl ester.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
ty in asymmetric catalysis.2 Recently, multidentate chiral NHC li-
gands presenting an anionic functional group have become a
useful strategy for constructing effective chiral environments in
proximity to the metal center. For instance, NHC ligands with hy-
droxy,3 sulfonate,4 or amide5 moieties in the N-substituent have
been introduced (Fig. 1), and these ligands enable highly enantio-
selective transformations with various transition metals. However
N-Heterocyclic carbenes (NHCs) have attracted considerable
interest both in organic and organometallic chemistry due to their
highly electron-donating and strong coordination abilities in form-
ing transition metal complexes.1 Therefore, NHC–transition metal
complexes exhibit a unique catalytic activity and enantioselectivi-
Ph
Ph
N
N
Mes
N
N
N
N
Mes
Me
O
S
HO
HO
O
O
Hoveyda (2002)
Ph
Arnold (2004)
Hoveyda (2007)
Ph
N
N
Mes
N
N
N
N
Mes
Me
HO
O
HO
HN
HO
Jung (2008)
Figure 1. Representative examples of chiral NHC ligands bearing an anionic functional group.
Hoveyda (2005)
Mauduit (2005)
⇑
Corresponding author. Tel.: +81 11 706 3434; fax: +81 11 706 3749.
E-mail address: sawamura@sci.hokudai.ac.jp (M. Sawamura).
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.05.013

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T. Iwai et al. / Tetrahedron: Asymmetry 24 (2013) 729–735
1) stereogenic centers
(B)
(A)
derived from a
chiral diamine
R2
R3
Ph
Me
Ph
4) sterically and
electronically
N
N
N
N
R1
Me
tunable substituent
Me
O
P
O
P
O
O
R4
2) global chiral
environment
O
O
O
O
3) phosphate-type
anionic coordination site
R5
1a
Figure 2. (a) Features of chiral NHC ligands 1 bearing a m-terphenyl-based phosphate moiety as an N-substituent. (b) Structure of 1a.
to date, a chiral NHC ligand bearing a diorganophosphate moiety
[(RO)2P(O)Oꢀ] as an anionic N-substituent has not been reported
in the literature despite the ubiquitous utilities of biarylphos-
phates in transition metal catalysis as counter anions of the metal6
or in organocatalysis as Brønsted acid–base components.7
We envisioned that the synthesis of chiral NHC ligands 1 bear-
ing a m-terphenyl-based phosphate moiety as an anionic N-substi-
tuent would be a useful strategy for developing efficient chiral
catalysts (Fig. 2a). The structural features of these chiral NHC li-
gands are as follows: (1) the NHC backbone has stereogenic centers
derived from easily-available chiral diamines; (2) the m-terphenyl
moiety as an N-substituent can provide global chiral environment
in proximity to the metal center; (3) the m-terphenyl-based diorg-
anophosphate moiety works as an anionic coordination site result-
ing in C,O-bidentate coordination; and (4) the steric and electronic
nature of the catalyst is tunable by the other N-substituent (R1).
Based on these considerations, we selected chiral NHC ligand 1a,
which has a non-substituted m-terphenyl-based phosphate moiety
(R4 = R5 = H), an N-mesityl group (R1 = 2,4,6-Me3-C6H2), and a 1,2-
diphenylethylenediamine-derived NHC core (R2 = R3 = Ph) as the
initial synthetic target (Fig. 2b). Details of the ligand synthesis,
the coordination behavior toward a Rh(I) complex, and application
to the Cu(I)-catalyzed enantioselective boron conjugate addition
are described below.
o-bromoiodobenzene
(1.2 eq)
Pd(PPh3)4 (3 mol%)
Br
1) nBuLi (1.1 eq)
MeO OMe
MeO OMe Br
MeO OMe Bpin
THF, –78°C, 1h
2) iPrO Bpin
K3PO4 (3 eq)
(1.1 eq)
–78°C to rt, 12 h
DMF, 80°C, 23 h
5 95%
3
4 75%
pin = pinacolato
Ph
Ph
(S)
(S)
Ph
Ph
Ph
Ph
HN
(1.5 eq)
H2N
NH2
MesBr (1.1 eq)
Pd(dba)2 (10 mol%)
rac-BINAP (20 mol%)
Mes NH HN
MeO OMe
H2N
Pd(OAc)2 (10 mol%)
rac-BINAP (20 mol%)
BBr3
MeO OMe
(6 eq)
NaOtBu (3 eq)
NaOtBu (3 eq)
toluene, 110°C, 28 h,
CH2Cl2
rt, 12 h
toluene, 110°C, 32 h
6 86 %
7
86 %
Mes = 2,4,6-Me3-C6H2
(rotamer 6:4)
(rotamer 6:4)
Ph
Ph
Ph
Ph
1) CH(OEt)3
(40 eq)
NH4BF4
(1.1 eq)
Ph
Ph
Mes NH HN
N
N
PCl2(O)(OMe)
(1.1 eq)
Mes
Mes NH HN
HO OH
O
P
–
O
BF4
O
MeO
O
P
O
Et3N (2.0 eq)
110 ºC, 10 h
MeO
O
CH2Cl2, rt, 13 h
8
94%
9
71 %
10
9a 38 % (rotamer 9:1)
9b 33 % (rotamer 7:3)
Ph
Ph
Ph
Ph
2) PhSH (0.9 eq)
Et3N (1.0 eq)
THF, rt, 13 h
CH(OEt)3
N
N
N
N
Mes
Mes
BF4
(40 eq)
NH4BF4
(1.1 eq)
–
O
O
MeO
MeO
P
7
3) K2CO3 aq.
O
(rotamer 6:4)
O
CH2Cl2, rt, 0.5 h
110 ºC, 10 h
11 96%
(rotamer 1:1)
2
33%
(rotamer 9:1)
(in three steps)
Scheme 1. Synthesis of chiral imidazolinium salt 2 and 11 as a precursor to NHC. One of the possible rotamers is shown for 2, 10, and 11.

T. Iwai et al. / Tetrahedron: Asymmetry 24 (2013) 729–735
731
the desired chiral imidazolinium zwitterion 2 bearing a diorgano-
phosphate moiety in a useful yield. The two rotamers of 2 were ob-
servable in a 9:1 ratio by 1H NMR spectroscopy in CDCl3 at room
temperature. Although definitive structure determination for the
major rotamer has yet to be successful, an analogy from Hoveyda’s
X-ray structures for N-biaryl-substituted NHC–metal complexes3g
suggests that the rotameric isomerization is due to rotation around
the C–C bond connecting the N-phenyl group and the biphenyl
phosphate unit and that the major rotamer is the one in which
the biphenyl phosphate unit is anti to the phenyl group at the near-
by stereogenic center of the imidazolinium ring as depicted in
Scheme 1. We also synthesized di-MeO-substituted imidazolinium
salt 11, which existed as a 1:1 rotamer mixture, as a reference
compound in order to evaluate an effect of the phosphate moiety
of 2 by treating 7 with CH(OEt)3/NH4BF4.
2. Results and discussion
The synthesis of imidazolinium zwitterion 2, which is a precur-
sor to the NHC ligand 1a, is outlined in Scheme 1. Halogen/lithium
exchange of 38 followed by treatment with isopropoxyboronic acid
pinacol ester gave 4 in 75% yield. The Pd-catalyzed Suzuki–Miyaura
cross-coupling of 1-bromo-2-iodobenzene with 4 afforded m-ter-
phenyl bromide 5 in excellent yield. For the preparation of the
unsymmetrical diamine and imidazolinium salt, Hoveyda’s proce-
dures for the synthesis of chiral bidentate NHCs were followed
with
a
slight modification.3g Thus, commercially available
(1S,2S)-1,2-diphenylethylenediamine was coupled with 5 to give
monoarylated diamine 6 through Pd-catalyzed C–N coupling. The
1H and 13C NMR spectra of 6 at room temperature showed the exis-
tence of rotamers in a 6:4 ratio. The second C–N coupling of 6 with
mesityl bromide catalyzed by the Pd(dba)2/BINAP complex affor-
ded unsymmetrical diaryldiamine 7 in 86% yield. Next, demethyl-
ation of 7 with BBr3 gave the corresponding biphenol 8 in a
quantitative yield. The 1H and 13C NMR spectra of 8 did not show
the existence of rotamers.
Next, the reaction of 8 with dichlorophosphoric acid methyl es-
ter in the presence of Et3N gave organophosphate 9. The 31P NMR
spectrum of the crude product 9 showed four peaks (Fig. 3a), which
were assigned to signals of a pair of diastereomers, each consisting
of rotamers (9a with 9:1 rotamer ratios and 9b with 7:3 rotamer
ratios): the appearance of the diastereomers is due to the existence
of the P stereogenic center. The diastereomers 9a and 9b could be
separated by silica gel column chromatography for characteriza-
tion by 1H, 13C, and 31P NMR spectroscopy (Fig. 3b). Construction
of the imidazolinium ring was performed by treatment of the dia-
stereomer mixtures of 9 (9a and 9b) with CH(OEt)3/NH4BF4 to give
imidazolinium salt 10. Finally, deprotection of the phosphate
methyl ester followed by anion exchange with K2CO3 afforded
The coordination behavior of 1a toward a Rh(I) complex was
investigated through the reaction between 2 and [RhCl(cod)]2 in
the presence of KOtBu in THF (Scheme 2). Air- and moisture-stable
dimeric Rh(I) complex [Rh(1a)(cod)]2 was obtained in 64% yield
after silica gel chromatography. The ESI mass spectrum showed
the dimeric nature of this Rh(I) complex (Fig. 4a). The 1H, 13C,
and 31P NMR spectroscopic analyses indicated that the two rota-
mers of 2 converged with a single stereoisomer. A doublet signal
2
due to a P-Rh coupling in the 31P NMR spectrum (d 5.8, JRh-
P = 8.9 Hz in Fig. 4b) indicates that the diorganophosphate moiety
on 1a has a direct interaction with the rhodium center. Thus, a
C,O-bidentate coordination of 1a was confirmed. Similar dimeric
NHC–metal complexes with an anionic N-substituent have been
reported by Hoveyda3g,4a and Jung.5a
In order to evaluate the utility of the chiral NHC ligand 1a bear-
ing a phosphate moiety, we examined the copper-catalyzed enan-
tioselective boron conjugate addition to an
a,b-unsaturated
carbonyl compound.9 Although this enantioselective catalysis has
Figure 3. 31P NMR spectra of (a) crude product 9 and (b) 9a purified by silica gel
chromatography. Filled (d) and open circles (s) indicate 9a and 9b, each having
two rotamers, respectively.
Figure 4. (a) ESI-Mass and (b) 31P NMR (CDCl3) spectra of [Rh(1a)(cod)]2.
Ph
Ph
[RhCl(cod)]2
(1.0 eq)
O
N
N
O
O
Mes
N
Mes
P
Ph
Ph
Ph
Ph
N
O
O
KOtBu (2.0 eq)
O
Rh
Rh
P
O
O
N
N
THF, rt, 1h
64%
O
O
P
Mes
O
O
2
(2.0 eq)
cod
=
[Rh(1a)(cod)]2
Scheme 2. Synthesis of [Rh(1a)(cod)]2.

732
T. Iwai et al. / Tetrahedron: Asymmetry 24 (2013) 729–735
Cu salt (3.0 mol%)
imidazolinium salt
(6.0 mol%)
O
O
O
O
O
O
O
B
O
KOtBu (30 mol %)
+
B
B
Ph
OMe
Ph
OMe
Ph
MeOH (2.0 eq)
12
13 (1.1 eq)
THF, –55ºC, 24 h
14
imidazolinium salt
Cu salt, enantioselectivity and isolated yield of 14
Ph Ph
Ph
Ph
Ph
N
N
N
N
N
N
Mes
Mes
Mes
Mes
O
BF4
15
BF4
O
MeO
MeO
P
O
O
2
11
(rotamer 9:1)
(rotamer 1:1)
89% ee
86% yield
96% ee
94% yield
32% ee
83% yield
CuOtBu
77% ee
87% ee
31% ee
CuI
88% yield
73% yield
82% yield
79% ee
83% yield
95% ee
72% yield
10% ee
83% yield
CuCl2•2H2O
Scheme 3. Ligand effects of copper-catalyzed enantioselective boron conjugate addition to 12. Reaction conditions: 12 (0.20 mmol), 13 (0.22 mmol), Cu salt (0.0060 mmol,
3.0 mol %), imidazolinium salt (0.012 mmol, 6.0 mol %), KOtBu (0.060 mmol, 30 mol %), MeOH (0.40 mmol), THF (1.0 mL), ꢀ55 °C, 24 h.
mainly been developed by studies with phosphine-based chiral li-
gands,10 recent work by McQuade and Hoveyda has shown that
some chiral NHC ligands are useful for obtaining high catalyst turn-
over efficiencies or a broad substrate scope.11 Accordingly, we ex-
pected that 1a may show favorable effects for these reactions.
ester, the m-terphenyl-based phosphate moiety caused a favorable
ligand effect in comparison with a simple N-aryl substituent such
as a mesityl group. Further investigation into the development of
phosphate-based chiral NHC systems useful for enantioselective
catalysis is currently ongoing.
The ligand effects of several chiral NHCs in the reaction of a,b-
unsaturated ester 12 and bis(pinacolato)diboron 13 in the presence
of MeOH as a proton source and a catalytic amount of CuOtBu are
summarized in Scheme 3. The boron conjugate addition reaction
with the chiral imidazolinium zwitterion 2 bearing the diorgano-
phosphate afforded the desired product 14 with 89% ee (R) in
86% yield. The di-MeO-substituted imidazolinium salt 11 was then
used as a precursor of the NHC ligand in order to evaluate the effect
of the phosphate functionality in 1a. Against our expectation, how-
ever, this reaction occurred in excellent enantioselectivity, giving
(R)-14 with 96% ee. On the other hand, conventional C2-symmetri-
cal chiral imidazolinium salt 1511a,12 afforded 14 with poor enanti-
oselectivity [32% ee (R)]. The use of air- and moisture-stable CuI
and CuCl2ꢁ2H2O instead of CuOtBu as a copper source resulted in
similar trends in their catalytic activities and enantioselectivities.
These results indicate that the heteroatom-functionalized m-ter-
phenyl structures in 2 and 11 are effective scaffolds for introducing
the N atom of NHC systems, producing effective chiral environ-
ments in proximity to the metal center. However, the effect of
the diorganophosphate as an anionic functional group in this enan-
tioselective catalysis is unclear.
4. Experimental
4.1. General
NMR spectra were recorded on a Varian Gemini 2000 spectrom-
eter, operating at 300 MHz for 1H NMR, 75.4 MHz for 13C NMR, and
121.4 MHz for 31P NMR. Chemical shift values for 1H, 13C, and 31P
NMR spectra are referenced to Me4Si, the residual solvent, and
85% phosphoric acid, respectively. High-resolution mass spectra
were recorded on a Thermo Fisher Scientific Exactive, JEOL JMS-
T100LP mass spectrometer or JEOL JMS-T100 GC mass spectrome-
ter at the Instrumental Analysis Division, Equipment Management
Center, Creative Research Institution, Hokkaido University. HPLC
analyses were conducted on a HITACHI ELITE LaChrom system
with a HITACHI L-2455 diode array detector. Melting points were
determined on a micro melting point apparatus (Yanaco: MP-
500D) using micro cover glass. TLC analyses were performed on
commercial glass plates bearing 0.25-mm layer of Merck Silica
gel 60F254. Silica gel (Kanto Chemical Co., Silica gel 60N, spherical,
neutral) was used for column chromatography.
All reactions were carried out under an argon or nitrogen atmo-
sphere. Materials were obtained from commercial suppliers or pre-
pared according to standard procedures unless otherwise noted.
3. Conclusion
We have developed a novel imidazolinium zwitterion 2 bearing
a diorganophosphate anionic functional group, as a precursor to a
chiral NHC ligand 1a. The 31P NMR spectrum of [Rh(1a)(cod)]2 indi-
cated that the phosphate moiety is capable of coordinating to the
metal center through a negatively charged O atom to form a C,O-
chelate complex. In preliminary studies for the copper-catalyzed
4.2. Preparation of imidazolinium zwitterion 2
4.2.1. Boronate 4
At first, nBuLi in hexane (1.6 M, 11.8 mL, 18.8 mmol, 1.1 equiv)
was added dropwise to a 200-mL two-necked flask of 3-bromo-
2,20-dimethoxy-1,10-biphenyl8 3 (5.01 g, 17.1 mmol, 1.0 equiv) in
enantioselective boron conjugate addition to an
a,b-unsaturated

T. Iwai et al. / Tetrahedron: Asymmetry 24 (2013) 729–735
733
THF (50 mL) at ꢀ78 °C and the mixture was stirred for 1 h at this
temperature. 2-Isopropoxyboronic acid pinacol ester (3.8 mL,
18.8 mmol, 1.1 equiv) was then added at ꢀ78 °C and the mixture
was allowed to warm to room temperature and stirred for 12 h.
After quenching with aq NH4Cl, the mixture was extracted with
Et2O. The organic layer was washed with water, dried over MgSO4,
filtered, and concentrated. The residue was purified by silica gel
chromatography (30:1 to 10:1 hexane/EtOAc) to give 4 as a white
solid (4.36 g, 75% yield). Mp: 119–121 °C. 1H NMR (CDCl3): d 1.38
(s, 12H), 3.52 (s, 3H), 3.77 (s, 3H), 6.94–7.00 (m, 2H), 7.17 (t,
J = 7.4 Hz, 1H), 7.33 (d, J = 7.4 Hz, 2H), 7.40 (dd, J = 7.7, 1.9 Hz,
1H), 7.76 (d, J = 7.5 Hz, 1H). 13C NMR (CDCl3): d 24.76 (4C), 55.52,
61.86, 83.53 (2C), 110.83, 120.23, 123.09, 127.72, 128.67, 131.85,
131.98, 135.23, 136.20, 156.97, 163.46. A signal for the carbon di-
rectly attached to the boron atom was not observed. HRMS-ESI (m/
z): [M+Na]+ calcd for C20H25Na10BO4, 362.17744; found,
362.17737.
calcd
for
C34H33N2O2,
501.25365;
found:
501.25302.
½ ꢂ
a 2D1
¼ ꢀ41:85 (c 1.07, CHCl3).
4.2.4. Diaryl diamine 7
At first, NaOtBu (548 mg, 5.70 mmol, 3.0 equiv) was placed in a
100-mL Schlenk flask and then dried under vacuum at 110 °C for
10 min. After cooling to room temperature, rac-BINAP (238 mg,
0.382 mmol, 0.2 equiv), Pd(dba)2 (110 mg, 0.191 mmol, 0.1 equiv),
and toluene (12 mL) were added and the resulting dark red solu-
tion was stirred for 0.5 h at room temperature. The monoaryl dia-
mine 6 (950 mg, 1.90 mmol, 1.0 equiv) and mesityl bromide
(0.31 mL, 2.05 mmol, 1.1 equiv) were added to the solution and
the mixture was stirred at 110 °C for 28 h. After cooling to room
temperature, the reaction mixture was diluted with CH2Cl2 and
the organic layer was washed with water, dried over MgSO4, fil-
tered, and concentrated. The residue was purified by silica gel
chromatography (50:1 to 10:1 hexane/EtOAc) to give 7 as a light
yellow solid (1.01 g, 86% yield). The title compound 7 existed as
a mixture of atropisomers and the ratio (6:4) was determined by
1H NMR analysis. Mp: 81–83 °C. 1H NMR (CDCl3): d 1.78 (s,
3.6H), 1.79 (s, 2.4H), 2.10 (s, 1.2H), 2.11 (s, 1.8H), 3.15 (s, 1.2H),
3.25 (s, 1.8H), 3.50 (br, 1H), 3.65 (s, 1.8H), 3.79 (s, 1.2 H), 4.25
(br, 1H), 4.74–4.80 (m, 1H), 5.54 (d, J = 5.0 Hz, 0.4H), 5.64 (br s,
0.6H), 6.38–6.46 (m, 1H), 6.58 (s, 2H), 6.69–6.90 (m, 4H), 6.98–
7.38 (m, 16H). 13C NMR (CDCl3): Due to the complexity of the spec-
tra, signal assignment based on the atropisomerism is difficult and
not shown here. d 19.14, 20.73, 55.58, 55.78, 60.92, 61.01, 63.12,
63.33, 67.67, 67.75, 110.81, 111.13, 112.25, 112.35, 117.29,
117.38, 120.63, 124.02, 124.23, 126.47, 126.57, 127.47–129.55
(m), 130.76, 131.02, 131.93, 132.13, 132.79, 133.16, 133.24,
141.29, 141.42, 141.56, 141.63, 141.87, 142.10, 145.34, 145.77,
156.46, 156.66, 157.13, 157.38. ESI-MS (m/z): [M+H]+ 619.33. Anal.
calcd for C43H42N2O2, C 83.46, H 6.84, N 4.53; found, C 83.56, H
4.2.2. Bromide 5
Boronate 4 (4.01 g, 11.8 mmol, 1.0 equiv), Pd(PPh3)4 (409 mg,
0.354 mmol, 0.03 equiv), and K3PO4 (7.51 g, 35.4 mmol, 3.0 equiv)
were placed in a 100-mL Schlenk flask and were dissolved in
DMF (52 mL) at room temperature. 2-Bromo-iodobenzene
(1.8 mL, 14.0 mmol, 1.2 equiv) was then added and the solution
was stirred at 80 °C for 23 h. After cooling to room temperature,
the reaction mixture was diluted with Et2O and the organic layer
was washed with water, dried over MgSO4, filtered, and concen-
trated. The residue was purified by silica gel chromatography
(50:1 to 10:1 hexane/EtOAc) to give 5 as a white solid (4.14 g,
95% yield). Mp: 104–105 °C. 1H NMR (CDCl3): d 3.16 (s, 3H), 3.81
(s, 3H), 6.97–7.04 (m, 2H), 7.17–7.25 (m, 3H), 7.32–7.42 (m, 5H),
7.68 (d, J = 8.0 Hz, 1H). 13C NMR (CDCl3): d 55.56, 60.46, 110.96,
120.40, 122.87, 123.94, 126.98, 127.74, 128.77, 128.81, 130.52,
131.39, 131.67, 131.75, 132.18, 132.64, 134.54, 140.08, 155.61,
156.93. ESI-MS (m/z): [M+H]+ 369.05. Anal. calcd for C20H17BrO2,
C 65.05, H 4.64; found: C 65.17, H 4.62.
6.96, N 4.40. ½a 2D4
¼ ꢀ89:2 (c 1.01, CHCl3).
ꢂ
4.2.5. Biphenol 8
Diaryl diamine 7 (1.00 g, 1.62 mmol) and CH2Cl2 (70 mL) were
added to a 200-mL two-necked flask and the solution was cooled
at 0 °C. Next, BBr3 in CH2Cl2 (1.0 M, 9.7 mL, 9.7 mmol, 6 equiv)
was added slowly and the solution was allowed to warm to room
temperature and stirred for 14 h. After quenching with aq. NaH-
CO3, the mixture was extracted with CH2Cl2. The organic layer
was washed with water, dried over MgSO4, filtered, and concen-
trated. The residue was purified by silica gel chromatography
(10:1 to 3:1 hexane/EtOAc) to give 8 as a light yellow solid
(895 mg, 94% yield). Mp: 89–92 °C. 1H NMR (CDCl3): d 1.75 (s,
6H), 2.10 (s, 3H), 4.16 (d, J = 9.6 Hz, 1H), 4.73 (d, J = 9.6 Hz, 1H),
6.56 (s, 2H), 6.82–6.89 (m, 2H), 6.90–7.41 (m, 19H). Signals for
the four protons (NH and OH) were not observed. 13C NMR (CDCl3):
Due to the complexity of the spectra, the signals are not fully as-
signed. d 18.60, 20.28, 63.21, 67.17, 114.40, 118.05, 118.36,
119.08, 120.81, 121.06, 121.79, 122.48, 127.12–129.74 (m),
131.20–132.97 (m), 134.55, 139.87 (br), 143.97 (br), 149.94,
154.38 (br), 162.09, 162.93. ESI-MS (m/z): [M+H]+ 591.29. Anal.
calcd for C41H38N2O2, C 83.36, H 6.48, N 4.74; found: C 83.07, H
4.2.3. Monoaryl diamine 6
At first, NaOtBu (1.56 g, 16.4 mmol, 3.0 equiv) was placed in a
100-mL Schlenk flask and was dried under vacuum at 110 °C for
10 min. After cooling to room temperature, rac-BINAP (718 mg,
1.15 mmol, 0.2 equiv), Pd(OAc)2 (122 mg, 0.54 mmol, 0.1 equiv),
and toluene (47 mL) were added and the resulting dark red solu-
tion was stirred for 0.5 h at room temperature. Bromide
5
(2.00 g, 5.42 mmol, 1.0 equiv) and (1S,2S)-(ꢀ)-diphenylethylenedi-
amine (1.73 g, 8.15 mmol, 1.5 equiv) were added to the solution
and the mixture was stirred at 110 °C for 32 h. After cooling to
room temperature, the reaction mixture was diluted with CH2Cl2
and the organic layer was washed with water, dried over MgSO4,
filtered, and concentrated. The residue was purified by silica gel
chromatography (3:1 to 2:1 hexane/EtOAc) to give 6 as a light yel-
low solid (2.33 g, 86% yield). Title compound 6 exists as a mixture
of atropisomers and the ratio (6:4) was determined by 1H NMR
analysis. Mp: 73–75 °C. 1H NMR (CDCl3): d 1.32 (br s, 2H), 2.99 (s
1.2H), 3.31 (s, 1.8H), 3.82 (s, 3H), 4.20 (d, J = 4.7 Hz, 0.4H), 4.25
(d, J = 3.6 Hz, 0.6H), 4.43 (br s, 0.6H), 4.50 (br s, 0.4H), 5.13 (d,
J = 5.8 Hz, 0.4H), 5.35 (d, J = 6.6 Hz, 0.6H), 6.19 (d, J = 8.0 Hz, 0.6
H), 6.30 (d, J = 8.3 Hz, 0.4 H), 6.61 (q, J = 7.2 Hz, 1H), 6.93–7.46
(m, 19H). 13C NMR (CDCl3): Due to the complexity of the spectra,
signal assignment based on the atropisomerism is difficult and
6.59, N 4.60. ½a 2D4
¼ ꢀ1:70 (c 1.01 CHCl3).
ꢂ
4.2.6. Phosphate 9
Biphenol 8 (900 mg, 1.52 mmol) and CH2Cl2 (20 mL) were
added to a 50-mL two-necked flask and the solution was cooled
at 0 °C. Next, methyl phosphorodichloridate (249 mg, 1.67 mmol,
1.1 equiv) was added slowly and the solution was allowed to
warm to room temperature and stirred for 13 h. After quenching
with aq. NaHCO3, the mixture was extracted with CH2Cl2. The or-
ganic layer was washed with water, dried over MgSO4, filtered,
and concentrated. The residue was purified by silica gel chroma-
not shown here.
d 55.52, 60.31, 60.76, 61.19, 63.29, 63.82,
110.90, 111.08, 111.49, 115.95, 116.36, 120.32, 120.34, 123.57,
123.61, 124.73, 125.30, 126.92, 127.02, 127.09, 127.14, 128.02–
128.74 (m), 130.26 130.54, 131.28, 131.32, 131.53, 131.78,
132.04, 132.22, 132.48, 132.55, 132.97, 141.37, 141.90, 142.59,
142.82, 144.40, 144.47, 156.15, 157.00. HRMS-ESI (m/z): [M+H]+

734
T. Iwai et al. / Tetrahedron: Asymmetry 24 (2013) 729–735
tography (15:1 to 5:1 hexane/EtOAc) to give diastereomeric
phosphates 9a (eluted first, 385 mg, 38%) and 9b (eluted later,
335 mg, 33%). Each diastereomer 9 (a light yellow solid) existed
as a mixture of atropisomers. The ratios (9a, 9:1; 9b, 7:3) were
determined by 1H NMR analysis. 9a: Mp: 115–118 °C. 1H NMR
(CDCl3): d 1.43 (s, 5.4H), 1.77 (s, 0.6H), 2.11 (s, 0.3H), 2.13 (s,
2.7H), 3.22 (br d, J = 12.1 Hz, 1H), 3.31 (d. J = 11.8 Hz, 0.3H),
3.48 (d, J = 11.8 Hz, 2.7H), 3.75 (t, J = 10.9 Hz, 1H), 4.60 (d,
J = 9.6 Hz, 1H), 6.23 (s, 1H), 6.32 (d, J = 8.0 Hz, 1H), 6.43 (s, 2H),
6.56 (t, J = 7.2 Hz, 1H), 6.76 (t, J = 7.4 Hz, 1H), 6.85–6.92 (m,
3H), 6.97–7.60 (m, 15H). 13C NMR (CDCl3): Due to the complexity
of the spectra, signal assignment based on the atropisomerism
and C–P couplings is difficult and not shown here. d 18.62,
20.22, 55.08, 55.28, 60.41, 60.50, 62.61, 62.81, 67.15, 67.21,
110.29, 110.62, 111.74, 111.83, 116.77, 116.85, 120.13, 123.50,
123.71, 125.95, 126.05, 126.96–129.03 (m), 129.52, 130.51,
131.28, 131.42, 131.62, 132.27, 132.62, 132.73, 140.77, 140.91,
141.05, 141.12, 141.36, 141.57, 144.83, 145.24, 155.96, 156.62,
156.87. 31P NMR (CDCl3): d 3.72 (minor), 3.13 (major). ESI-MS
(m/z): [M+H]+ 667.27. Anal. C42H39N2O4P, C 75.66, H 5.90, N
Due to the complexity of the spectra, signal assignment based on
the atropisomerism and C–P couplings is difficult and not shown
here. d 16.44, 16.90, 17.07, 18.99, 20.65, 20.71, 70.17, 74.38,
74.97, 75.24, 122.73, 122.78, 123.85, 124.09, 125.06, 128.10–
131.35 (m), 132.41–134.03 (m), 134.15, 135.24, 136.09, 136.97,
138.34, 138.78, 139.70, 148.84, 148.96, 149.14, 149.25, 151.36,
151.48, 151.62, 151.74, 161.04, 160.08, 161.50. 31P NMR (CDCl3):
d 5.94 (minor), 5.29 (major). HRMS-ESI (m/z): [M+Na]+ calcd for
C
42H35N2NaO4P, 685.22267; found, 685.22205. ½a D25
¼ þ19:55 (c
ꢂ
1.00, CHCl3).
4.2.8. Imidazolinium salt 11
Diaryl diamine 7 (50.3 mg, 0.081 mmol), CH(OEt)3 (0.54 mL,
3.23 mmol 40 equiv), and NH4BF4 (9.5 mg, 0.090 mmol 1.1 equiv)
were placed in a 10-mL Schlenk flask. The mixture was stirred at
110 °C for 10 h. After cooling to room temperature, the volatiles
were removed under vacuum. The residue was purified by silica
gel chromatography (CHCl3 then 20:1 CHCl3/MeOH) to give 11 as
a white solid (52.7 mg, 96% yield). The title compound 11 existed
as a mixture of atropisomers and the ratio (55:45) was deter-
mined by 1H NMR analysis. Mp: 139–143 °C. 1H NMR (CDCl3):
4.20; found, C 75.55, H 6.17, N 4.05. ½a D25
¼ ꢀ105:8 (c 1.01,
ꢂ
CHCl3). 9b: Mp: 212–213 °C. 1H NMR (CDCl3): d 1.65 (s, 3H),
1.72 (s, 3H), 2.10 (s, 3H), 3.14 (d, J = 11.6 Hz, 3H), 3.68 (d,
J = 11.6 Hz, 1H), 4.10 (br, 1H), 4.63 (br d, J = 8.2 Hz, 0.3H), 4.70
(dd, J = 8.8, 3.0 Hz, 0.7H), 5.47 (d, J = 2.8 Hz, 0.7H), 5.54 (br s,
0.3H), 6.36–6.42 (m, 1H), 6.49 (s, 0.6H), 6.52 (s, 1.4H), 6.69–
6.82 (m, 3H), 6.97–7.63 (m, 17H). 13C NMR (CDCl3): Due to the
complexity of the spectra, signal assignment based on the atrop-
isomerism and C–P couplings is difficult and not shown here. d
18.37, 18.61, 20.27, 20.29, 54.58, 54.64, 55.08, 55.14, 62.30,
63.27, 67.26, 67.31, 112.12, 112.29, 116.54, 117.56, 120.98,
121.03, 122.01, 122.06, 122.71, 123.63, 126.49–132.89 (m),
140.34, 140.37, 140.65, 140.68, 141.01, 141.60, 145.02, 145.38,
145.75, 145.90, 146.04, 146.08, 147.27, 147.37, 147.58. 31P NMR
(CDCl3): d 4.00 (major), 3.35 (minor). ESI-MS (m/z): [M+H]+
667.27. Anal. calcd for C42H39N2O4P, C 75.66, H 5.90, N 4.20;
d 1.54 (s, 1.35H), 1.76 (s, 1.65H), 2.16 (s, 1.35H), 2.19 (s,
1.65H), 2.33 (s, 1.65H), 2.57 (s, 1.35H), 3.17 (s, 1.35H), 3.21 (s,
1.65H), 3.27 (s, 1.35H), 3.74 (s, 1.65H), 5.39–5.48 (m, 1.45H),
6.17 (d, J = 10.2 Hz, 0.55H), 6.60–6.67 (m, 2H), 6.83–7.62 (m,
20H), 7.78 (d, J = 8.1 Hz, 0.55H), 7.93 (d, J = 8.2 Hz, 0.45H), 8.54
(s, 0.55H), 8.59 (s, 0.45H). 13C NMR (CDCl3): Due to the complex-
ity of the spectra, signal assignment based on the atropisomerism
is difficult and not shown here. d 17.81, 18.24, 18.64, 20.73,
54.75, 55.10, 60.97, 61.03, 72.31, 72.62, 75.13, 75.26, 110.72,
110.89, 120.08, 120.53, 124.22, 124.67, 126.63, 126.71, 128.00,
128.57–130.63 (m), 131.08, 131.18, 131.21, 131.53, 131.88,
131.98, 132.07, 132.32, 132.64, 133.27, 133.29, 133.44, 133.65,
134.46, 134.81, 134.91, 134.99, 135.20, 135.79, 135.85, 136.07,
140.04, 140.24, 155.64, 155.73, 156.72, 156.75, 157.14, 157.90.
HRMS-ESI (m/z): [MꢀBF4]+ calcd for C44H41N2O2, 629.31626;
found: C 75.22, H 5.82, N 4.19. ½a D25
ꢂ
¼ ꢀ2:05 (c 1.01, CHCl3).
found: 629.31556. ½a D25
¼ ꢀ227:0 (c 1.02, CHCl3).
ꢂ
4.2.7. Imidazolinium zwitterion 2
4.3. Preparation of [Rh(1a)(cod)]2 (Scheme 2)
Phosphate 9 (a mixture of 9a and 9b, 300 mg, 0.45 mmol),
CH(OEt)3 (3.0 mL, 18 mmol, 40 equiv), and NH4BF4 (52.0 mg,
0.50 mmol, 1.1 equiv) were placed in a 10-mL Schlenk flask. The
mixture was stirred at 110 °C for 10 h. After cooling to room tem-
perature, the volatiles were removed under vacuum. The residue
was passed through a short pad of silica gel to give 10 containing
impurities. The mixture was placed in a 10-mL Schlenk flask and
dissolved in THF (1.0 mL). Next, PhSH (0.042 mL, 0.41 mmol,
0.90 equiv) and Et3N (0.069 mL, 0.45 mmol, 1.0 equiv) were added
and the resulting mixture was stirred at room temperature for
13 h. After quenching with H2O, the mixture was extracted with
CH2Cl2. The organic layer was washed with water, dried over
MgSO4, filtered, and concentrated. The residue was purified by sil-
ica gel chromatography (CHCl3 then 10:1 CHCl3/MeOH). Next, the
material was dissolved in CH2Cl2 (10 mL) and satd K2CO3 aq
(10 mL) was added. The resulting mixture was stirred vigorously
at room temperature for 0.5 h. The mixture was then extracted
with CH2Cl2 and the organic layer was washed with water, dried
over MgSO4, filtered, and concentrated to give 2 as a light yellow
solid (106.9 mg, 33% yield in three steps). The title compound 2 ex-
isted as a mixture of atropisomers and the ratio (9:1) was deter-
mined by 1H NMR analysis. Mp: >250 °C. 1H NMR (CDCl3): d 1.15
(s, 3H), 2.10 (s, 3H), 2.48 (s, 3H), 5.27 (d, J = 13.5 Hz, 0.2H), 5.52
(q, J = 9.4 Hz, 1.8H), 6.23 (s, 0.1H), 6.50 (s, 0.9H), 6.67 (s, 0.1H),
6.79 (s, 0.9H), 6.81 (d, J = 7.9 Hz, 1H), 6.98 (d, J = 7.1 Hz, 2H),
7.11–7.64 (m, 17H), 7.77 (dd, J = 7.4, 1.6 Hz, 0.9H), 7.94 (d,
J = 8.2 Hz, 0.1 H), 10.22 (s, 0.1H), 10.54 (s, 0.9H). 13C NMR (CDCl3):
At first, [RhCl(cod)]2 (11.2 mg, 0.023 mmol, 1.0 equiv) and
KOtBu (5.1 mg, 0.045 mmol, 2.0 equiv) were placed in a 5-mL test
tube with a screw cap. Next, THF (0.6 mL) was added to the ves-
sel and the mixture was stirred at room temperature for 1 h. Imi-
dazolium salt 2 (30.0 mg, 0.045 mmol, 2.0 equiv) in THF (0.4 mL)
was then added and the mixture was stirred at room tempera-
ture for 2 h. The volatiles were removed under vacuum, and
the residue was purified by preparative TLC (10:1 CHCl3/MeOH)
to give [Rh(1a)(cod)]2 as a yellow solid (25.4 mg, 64%). Mp:
210–215 °C (decomp). 1H NMR (CDCl3): d 1.39–1.62 (m, 7H),
1.71–1.84 (m, 3H), 1.89 (s, 6H), 1.93–2.13 (m, 4H), 2.16 (s, 6H),
2.23 (s, 6H), 2.28–2.63 (m, 2H), 2.63 (br, 2H), 3.49 (br, 2H),
4.61 (d, J = 5.2 Hz, 2H), 4.82 (br, 2H), 4.99 (br, 2H), 5.75 (d,
J = 7.1 Hz, 2H), 6.36 (d, J = 7.1 Hz, 4H), 6.70–6.85 (m, 8H), 7.05
(t, J = 7.6 Hz, 2H), 7.17–7.50 (m, 26H), 7.62 (dd, J = 7.4, 1.6 Hz,
2H), 7.74 (dd, J = 7.4, 1.6 Hz, 2H), 8.06 (d, J = 8.0 Hz, 2H). 13C
NMR (CDCl3): Due to the complexity of the spectra, signal assign-
ment based on C–P couplings is difficult and not shown here. d
19.18, 19.25, 20.75, 27.38, 27.50, 32.18, 32.45, 63.30, 63.53,
66.15, 66.37, 75.19, 98.09, 98.18, 98.76, 98.86, 122.54, 122.59,
123.95, 124.53, 127.07, 127.57, 127.76, 128.50–128.70 (m),
129.27–130.26 (m), 131.82, 132.58, 132.99, 133.02, 134.58,
134.69, 136.32, 137.81, 138.13, 138.96, 139.09, 139.93, 148.81,
148.94, 150.81, 150.92, 211.38, 212.01. 31P NMR (CDCl3): d 5.74
2
(d, JP-Rh = 8.9 Hz). ESI-MS (m/z): [M+H]+ 1745.44, [M-Rh-1a-
cod+H]+ 873.22. ½a 2D5
¼ ꢀ50:3 (c 1.02, CHCl3).
ꢂ

T. Iwai et al. / Tetrahedron: Asymmetry 24 (2013) 729–735
735
Garber, S. B.; Kataoka, O.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125, 12502; (d)
Arnold, P. L.; Rodden, M.; Davis, K. M.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C.
Chem. Commun. 2004, 1612; (e) Larsen, A. O.; Leu, W.; Oberhuber, C. N.;
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Coutable, L.; Guillemin, J.-C.; Mauduit, M. Tetrahedron: Asymmetry 2005, 16,
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4.4. Typical procedures for the catalytic reactions
Methyl cinnamate 12 (0.20 mmol) and bis(pinacolato)diboron
13 (0.22 mmol) were placed in a vial containing a magnetic stirrer
bar, and the vial was sealed with a TeflonÒ-coated silicon rubber
septum. Next, THF (0.80 mL) and MeOH (0.40 mmol) were added
to the vial, and the mixture was cooled at ꢀ55 °C. A Cu salt
(0.0060 mmol), an imidazolinium salt (0.012 mmol), and KOtBu
(0.060 mmol) were then placed in another vial with a TeflonÒ-
coated silicon rubber septum. After THF (0.20 mL) was added to
the vial, the mixture was stirred at 25 °C for 15 min. Next, the
Cu–NHC complex solution was transferred to the vial containing
12 and 13 at ꢀ55 °C. After 24 h stirring at ꢀ55 °C, the reaction mix-
ture was filtered through silica gel and washed with Et2O. The fil-
trate was concentrated under vacuum and the residue was purified
by silica gel column chromatography (20:1 hexane/EtOAc) to af-
ford the desired product 14. 1H NMR (CDCl3): d 1.17 (s, 6H), 1.22
(s, 6H), 2.63–2.76 (m, 2H), 2.86–2.94 (m, 1H), 3.65 (s, 3H), 7.13–
7.29 (m, 5H). 13C NMR (CDCl3): d 24.34 (2C), 24.43 (2C), 28.20
(br), 37.00, 51.49, 83.54 (2C), 125.76, 128.23 (2C), 128.56 (2C),
4. (a) Brown, M. K.; May, T. L.; Baxter, C. A.; Hoveyda, A. H. Angew. Chem., Int. Ed.
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141.37, 173.97. ½a D24
¼ ꢀ17:5 (c 1.01, CHCl3). The ee value was
ꢂ
determined by chiral HPLC analysis [CHIRALCELÒ OJ-H column,
4.6 mm ꢃ 250 mm, Daicel Chemical Industries, hexane/2-propa-
nol = 97:3, 0.50 mL/min, 40 °C, 220 nm UV detector, retention
time = 10.60 min for the (R)-isomer and 11.86 min for the
(S)-isomer]. The absolute configuration of 14 was assigned accord-
ing to the literature.10h
7. (a) Akiyama, T.; Itoh, J.; Fuchibe, K. Adv. Synth. Catal. 2006, 348, 999; (b)
Akiyama, T. Chem. Rev. 2007, 107, 5744; (c) Terada, M. Bull. Chem. Soc. Jpn. 2010,
83, 101.
8. Baret, P.; Béguin, C.; Gaude, D.; Gellon, G.; Mourral, C.; Pierre, J.-L.; Serratrice,
G.; Favier, A. Tetrahedron 1994, 50, 2077.
Acknowledgments
9. (a) Schiffner, J. A.; Müther, K.; Oestreich, M. Angew. Chem., Int. Ed. 2010, 49,
1194. For reviews, see; (b) Hartmann, E.; Vyas, D. J.; Oestreich, M. Chem.
Commun. 2011, 47, 7917.
This work was supported by Grants-in-Aid for Scientific Re-
search on Challenging Exploratory Research, MEXT and ACT-C,
JST to M.S and by Grant-in-Aid for Young Scientists (Start-up), JSPS
to T.I.
10. (a) Mun, S.; Lee, J.-E.; Yun, J. Org. Lett. 2006, 8, 4887. For the enantioselective
copper-catalyzed boron conjugate addition with phosphine- or amine-based
chral ligands, see; (b) Lee, D.; Kim, D.; Yun, J. Angew. Chem., Int. Ed. 2006, 45,
2785; (c) Lee, J.-E.; Yun, J. Angew. Chem., Int. Ed. 2008, 47, 145; (d) Fleming, W.
J.; Müller-Bunz, H.; Lillo, V.; Fernández, E.; Guiry, P. J. Org. Biomol. Chem. 2009,
7, 2520; (e) Feng, X.; Yun, J. Chem. Commun. 2009, 6577; (f) Feng, X.; Yun, J.
Chem. Eur. J. 2010, 16, 13609; (g) Solé, C.; Whiting, A.; Gulyás, H.; Fernández, E.
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Fernández, E. Organometallics 2009, 28, 659. For enantioselective copper-
catalyzed boron conjugate additions with chiral NHC ligands, see; (b) Park, J.
K.; Lackey, H. H.; Rexford, M. D.; Kovnir, K.; Shatruk, M.; McQuade, D. T. Org.
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