Chiral Bicyclic NHC/Cu Complexes for Catalytic Asymmetric Borylation of α,β-Unsaturated Esters

Article abstract of DOI:10.1021/acs.joc.9b02096

The potential of using chiral bicyclic NHC ligands that exhibit modularity was investigated in the Cu-catalyzed asymmetric borylation reaction of α,β-unsaturated esters. After screening for ligands and optimization of the reaction conditions, the corresponding products were afforded with good enantioselectivities (up to 85% ee).

Full text of DOI:10.1021/acs.joc.9b02096

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Note
Chiral Bicyclic NHC/Cu Complexes for Catalytic
Asymmetric Borylation of #,#-Unsaturated Esters
Yuya Miwa, Takumi Kamimura, Kiyoaki Sato, Daichi Shishido, and Kazuhiro Yoshida
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02096 • Publication Date (Web): 08 Oct 2019
Downloaded from pubs.acs.org on October 15, 2019
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Page 1 of 6
The Journal of Organic Chemistry
1
2
3
4
5
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8
9
Chiral Bicyclic NHC/Cu Complexes for Catalytic Asymmetric Borylation
of ,-Unsaturated Esters
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1
1
1
*1,2
Yuya Miwa, Takumi Kamimura, Kiyoaki Sato, Daichi Shishido, and Kazuhiro Yoshida
1
2
Department of Chemistry, Graduate School of Science and Molecular Chirality Research Center, Chiba University,
Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
E-mail: kyoshida@faculty.chiba-u.jp
*
ABSTRACT: The potential of using chiral bicyclic NHC ligands that exhibit modularity was investigated in the Cu-catalyzed
asymmetric borylation reaction of ,-unsaturated esters. After screening for ligands and optimization of the reaction con-
ditions, the corresponding products were afforded with good enantioselectivities (up to 85% ee).
A quarter of a century has passed since immense attention has
which imidazolium salts 2 are mixed with CuCl and t-BuOK in
THF (conditions II). Although our ligands are characterized by
10
been devoted to N-heterocyclic carbenes (NHCs) as strong -
1
donor ligands for transition metal catalysis. NHC ligands have
having variations at substituents R (at the chiral center) and R’
(on the nitrogen atom), the preparation of 4 with a diarylmethyl
or benzyl group (u–x) as the R’ substituent must be performed
under conditions I. On the other hand, because the preparation of
4 with an aryl group (y and z) as the R’ substituent is acceptable
under either conditions, we preferentially employed the simpler
conditions II.
often been compared with phosphine ligands, and catalytic reac-
tions in which the former show better performance than the latter
have become commonplace. However, when the range of ligands
2
is narrowed to asymmetric ones, there are no chiral NHCs that
show superior performance comparable to privileged phosphine
3
ligands, such as BINAP and Josiphos. Therefore, efforts to devel-
op chiral NHC ligands from various perspectives are still required
in this field.
Scheme 2. Synthesis and Structures of Chiral Bicyclic
NHC/Cu Complexes 4.
In recent years, our group has conducted research and devel-
opment of optically active bicyclic NHC ligands that have a rigid
4
and effective chiral environment (Scheme 1). An important fea-
ture of our previous study is the modularity of the ligand, which is
realized by combining various chiral imidazoles 1 and electro-
philes (R’–X). By utilizing this feature effectively, we developed an
excellent catalyst precursor for the Ir-catalyzed asymmetric
transfer hydrogenation of ketones (ee values of up to 98% and
4b
turnover numbers of up to 4,500). Currently known as the cata-
lyst precursor containing a monodentate chiral NHC ligand, it
shows the highest performance for this transformation.5
Scheme 1. Conversion of Chiral Bicyclic Imidazoles 1 into
Chiral Bicyclic NHC/Metal Complexes 3.
In this study, we applied the same system using our ligands to
the Cu-catalyzed asymmetric borylation reaction of ,-
unsaturated esters, in which NHCs are known to act as monoden-
6,7
tate chiral ligands effectively. Herein we describe the details of
the results.
Two synthetic procedures were employed for the preparation
of desired NHC/Cu complexes 4 (Scheme 2). One is the carbene
transfer approach from NHC/Ag complexes that are generated by
the in situ deprotonation of imidazolium salts 2 using Ag
ditions I), and the other is a more straightforward approach in
2
O (con-
8,9
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Page 2 of 6
With Cu complexes 4 in hand, catalyst screening for the asym-
metric borylation reaction of ,-unsaturated esters was per-
formed (Table 1), where ethyl cinnamate (5a) as the model sub-
all in terms of both yield and enantioselectivity (entries 1 vs 2-4).
When the temperature was decreased from –20 °C to –40 °C, little
improvement of the enantioselectivity was observed (entry 1 vs.
5). When the temperature was decreased further to –80 °C, not
only isolated yield but also selectivity deteriorated (entry 6).
Then, we fixed the temperature at –40 °C and examined the
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
strate was reacted with (Bpin)
2
(2 equiv) in the presence of Cu
catalyst (5 mol %), Cs CO (5 mol %), and methanol (2 equiv) in
2
3
11
THF at –20 °C for 6 h. Unexpectedly, under those conditions, the
desired transformation proceeded with even NHC-free CuCl as the
catalyst to give racemic alcohol 6a after routine oxidative workup
choice of base (entries 2, 7-10). We found that replacing Cs
2
CO
3
with t-BuONa or t-BuOK improved the enantioselectivity (entries
9 and 10). In regard to the reaction time, prolongation of the time
increased the yield slightly but decreased enantioselectivity (en-
tries 10–13). Therefore, considering the balance of yield and se-
lectivity, we chose 3 h as the reaction time. Then, we examined
the catalyst loading. Whereas decreasing it from 5 mol % to 2
mol % led to a negative outcome in terms of both yield and enan-
tioselectivity (entry 11 vs. entry 14), the enantioselectivity re-
mained unchanged when the catalyst loading was increased to 10
mol % (entry 11 vs. entry 15). From these results, we decided that
5 mol % is appropriate. Finally, the effect of additive was investi-
gated (entries 16 and 17). As the use of 1 equiv of t-BuOK with
molecular sieve gave a meaningful outcome in the isolated yield,
this was determined as the optimum condition (entry 17).
with NaBO
3
•4H O (entry 1). In regard to NHC/Cu complexes 4,
2
first, by fixing a diphenylmethyl group (u) as the R’ substituent,
the influence of the steric hindrance of the R substituent (a–d)
was examined (entries 2–5). As a result, complexes 4bu having a
mesityl group (b) and 4cu having a 2,6-diisopropylphenyl group
(c) showed preferable enantioselectivities (entries 3 and 4). Then,
we investigated the influence of the R’ substituent on the nitrogen
atom of NHC, where the R substituent was fixed to the 2,6-
diisopropylphenyl group (c) (entries 4 and 6–10). The results
demonstrated that aryl groups (y and z) made positive contribu-
tions to the enantioselectivity. Complexes 4cy having a phenyl
group (y) and 4cz having a mesityl group (z) showed 60% ee or
higher selectivity (entries 9 and 10). Finally, we tested combina-
tions of a mesityl group (b) as the R substituent and an aryl group
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
4
6
y or z) as the R’ substituent (entries 11 and 12), and found that
by was the best catalyst for this screening, giving a product with
9% ee (entry 11).
Table 2. Optimization of Reaction Conditions with (R)-
4by.
a
Table 1. Catalyst Screening for Cu-catalyzed Enantioselec-
tive -Borylation of ,-Unsaturated Ester.a
en-
try alys
cat-
solvent
base tem time
yiel ee
d
(%)
b
c
p
(h)
(mol %)
t
load
ing
(°C)
(
%
)
b
c
entry
catalyst
CuCl
yield (%)
ee (%)
(
mo
1
2
3
4
5
6
7
8
9
23
47
72
67
62
70
23
62
51
28
77
34
-
36
41
44
30
51
34
48
62
60
69
47
l %)
(R)-4au
(R)-4bu
(R)-4cu
(R)-4du
(R)-4cv
(R)-4cw
(R)-4cx
(R)-4cy
(R)-4cz
(R)-4by
(R)-4bz
1
2
3
4
5
6
7
8
9
0
1
2
5
THF
Et
toluene
CH CN
Cs
Cs
Cs
Cs
Cs
Cs
K
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
(5) –20
(5) –20
(5) –20
(5) –20
(5) –40
(5) –80
(5) –40
6
6
77 69
76 58
65 63
56 43
65 71
42 66
71 63
81 71
54 77
61 77
57 78
48 78
66 73
44 69
68 78
70 78
5
2
O
5
6
5
3
6
5
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
6
5
6
5
6
5
EtONa (5) –40
t-BuONa (5) –40
t-BuOK (5) –40
t-BuOK (5) –40
t-BuOK (5) –40
t-BuOK (5) –40
t-BuOK (5) –40
t-BuOK (5) –40
6
1
1
0
1
5
6
1
1
1
5
6
12
5
3
a
The reaction was carried out with ethyl cinnamate (5a)
and (Bpin) (2 equiv) in the presence of (R)-4 (5 mol %),
(5 mol %), and methanol (2 equiv) in THF at –20 °C for
5
1
2
13
5
15
3
Cs
6
2
CO
3
b
c
h. Isolated yield. Determined by HPLC analysis using a
1
1
1
4
5
6
2
chiral stationary phase column (Chiralcel OD-H).
10
5
3
t-BuOK –40
3
Using Cu complex 4by, optimization of the reaction conditions
was carried out (Table 2). In order to facilitate comparison of
results, the best conditions in Table 1 (entry 11) are shown again
in Table 2 (entry 1). Initial efforts focused on screening of sol-
vents (entries 2-4). However, THF was found to be the best after
(100)
1
7d
5
THF
t-BuOK –40
3
75 79
(
100)
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The Journal of Organic Chemistry
a
The reaction was carried out with ethyl cinnamate (5a)
BuOK (1 equiv), molecular sieve 3A, and methanol (2 equiv) in
THF at –40 °C for 3 h. Isolated yield. Determined by HPLC
b
c
1
2
3
4
5
6
7
8
9
and (Bpin)
2
(2 equiv) in the presence of (R)-4by (2–10 mol
b
%), base (5–100 mol %), and methanol (2 equiv) in THF. Iso-
lated yield. Determined by HPLC analysis using a chiral sta-
tionary phase column (Chiralcel OD-H) . Molecular sieve 3A
was added.
analysis using a chiral stationary phase column (Chiralcel OD-
c
d
H, Chiralpak AD-H, or Chiralpak AS-H). Determined by HPLC
d
analysis after conversion into the corresponding benzoate
using a chiral stationary phase column (Chiralpak AD-H).
Then, adopting the above conditions, the Cu-catalyzed asym-
metric -borylation reaction of several ,-unsaturated esters
was accomplished (Table 3). The data in entry 1 are identical to
those in entry 17 in Table 2, in which ethyl cinnamate (5a) was
employed as the substrate. It has become apparent that the size of
the ester moiety of the substrate affects the enantioselectivity.
When bulkier isopropyl cinnamate 5b was used, decreased enan-
tioselectivity was observed (entry 2). In contrast, when less bulky
methyl cinnamate 5c was used, a product with increased enanti-
oselectivity was obtained (entry 3). Then, we standardized the
Proposed models rationalizing the enantioselectivities of the
asymmetric reaction with (R)-4by is shown in Figure 1, where
disfavored and favored approaches of methyl cinnamate (5c) to
an NHC/Cu intermediate are shown. To avoid a steric interaction
between the phenyl ring on the nitrogen atom of NHC and the
carbomethoxy group of 5c, it is likely that the substrate favorably
coordinates to the Cu intermediate with its Si-face. There may
also be - stacking, stabilizing interaction between the phenyl
ring on the nitrogen atom of NHC and the phenyl ring of 5c to
contribute the favored approach.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
ester group to a methyl group and investigated the effect of R
group (entries 4–11). When a methyl group was introduced to the
aryl group at para- or meta-position, the results were comparable
to that of unsubstituted substrate 5c (entry 3 vs. entries 4 and 5).
However, use of sterically demanding ortho-substituted deriva-
tive 5f resulted in decreased enantioselectivity (entry 6). The
employment of other substituents and substitution patterns at the
para- and meta-positions of the aryl group did not cause serious
problems (entries 7–11). The best enantioselectivity (85% ee)
was observed for 5j and 5k having a trifluoromethyl group and a
chloro group at the para-position, respectively (entries 10 and
11). An attempt to expand the substrate scope to-alkyl-
substituted unsaturated ester did not succeed. When we conduct-
ed the reaction of 5l, a product with decreased enantioselectivity
was obtained (entry 12).
Table 3. Enantioselective -Borylation of ,-Unsaturated
a
Ester with (R)-4by.
Figure 1. Models Rationalizing the Enantioselectivity of -
Borylation of Methyl Cinnamate (5c) with (R)-4by.
In conclusion, we have applied optically active bicyclic NHC lig-
ands to the Cu-catalyzed asymmetric borylation reaction of ,-
unsaturated esters. After screening for the ligands and optimiza-
tion of the reaction conditions, corresponding nonracemic com-
pounds were obtained with good enantioselectivity (up to 85%
ee). Further modifications of the ligands and studies of their ap-
plication to other asymmetric reactions are in progress in our
laboratory.
R1
R2
yieldb
(%)
eec
(%
)
entry
product
1
2
Et
i-Pr
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Ph
Ph
Ph
(S)-6a
(S)-6b
(S)-6c
(S)-6d
(–)-6e
(–)-6f
(S)-6g
(–)-6h
(S)-6i
(S)-6j
(S)-6k
(S)-6l
75
83
69
88
87
76
89
89
79
46
89
79
73
81
81
84
75
84
83
83
85
85
3
EXPERIMENTAL SECTION
4
4-CH
3-CH
2-CH
3
3
3
C
C
C
6
6
6
6
6
H
H
H
H
4
4
4
4
General. All anaerobic and moisture-sensitive manipulations
were carried out with standard Schlenk techniques under
predried nitrogen or glove box techniques under prepurified ar-
5
6
1
gon. NMR spectra were recorded at 400 MHz for H and 100 MHz
13
1
7
3-ClC
3,5-(CH
for C{ H}. Chemical shifts are reported in δ ppm referenced to an
1
internal SiMe
4
standard for H NMR and chloroform-d (δ 77.0) for
8
3
)
2
C
H
3
13
1
C{ H} NMR. High-resolution mass spectra were recorded on
9
2-Naphthyl
Orbitrap mass spectrometers.
10
4-CF
3
C
6
H
4
Materials. 1,2-Dichloroethane and CH
2
Cl were distilled from
2
CaH under nitrogen and stored in a glass flask with a Teflon
2
1
1
1
2
4-ClC
6
H
4
stopcock under nitrogen. THF was distilled from sodium benzo-
phenone-ketyl under argon prior to use. MeOH was distilled from
magnesium under nitrogen and stored in a glass flask with a Tef-
lon stopcock under nitrogen. Imidazoles 1 were prepared accord-
i-Pr
73 68 d
a
The reaction was carried out with ,-unsaturated ester 5
and (Bpin) (2 equiv) in the presence of (R)-4by (5 mol %), t-
2
4
ing to the reported procedures or by slightly modified proce-
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Page 4 of 6
dures (vide infra). 1-Mesityl-3-(1-trityl-1H-imidazol-4-yl)propan-
1-ol, 2-alkyl-imidazolium salts 2 (u-x), and alkyl cinnamates 5
((R)-5-Mesityl-2-phenyl-6,7-dihydro-5H-pyrrolo[1,2-c]imidazol-
4b
4
12
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2-ylidene) copper chloride ((R)-4by); Following the General Pro-
cedure B-2 (conditions Ⅱ): (R)-2by (117.6 mg, 0.301 mmol), CuCl
(29.8 mg), t-BuOK (40 mg), molecular sieves 4A (90.3 mg), and
THF (3 and 9 mL) were used; purified by reprecipitation
were prepared according to the reported procedures. Diarylio-
donium tetrafluoroborates were prepared from the correspond-
ing aryl iodides and aryl boronic acids according to the reported
13
procedures. p-Toluenesulfonyl chloride, trimethylamine hydro-
(CH
2
Cl /hexane) to give (R)-4by (71.6 mg, 0.178 mmol, 59%
2
chloride, triethylamine, anhydrous DMF, Ag
2
O, Cu(OAc)
2
•H
2
O,
yield) (Although reprecipitation was repeated multiple times, it
CuCl, Cs CO , t-BuOK, t-BuONa, (Bpin) , and NaBO
2
3
2
3
•4H O were
2
was difficult to obtain as a completely pure product suitable for
1
elemental analysis.); H NMR (400 MHz, CDCl
3
) δ = 1.79 (s, 3H),
used as received. Molecular sieve 3A and 4A were predried in a
microwave oven before use.
Procedures for the Preparation of (R)-5-Mesityl-6,7-dihydro-5H-
pyrrolo[1,2-c] imidazole ((R)-1b) (modified procedures). A mix-
ture of 1-mesityl-3-(1-trityl-1H-imidazol-4-yl)propan-1-ol (1.58 g,
2
=
.27 (s, 3H), 2.52 (s, 3H), 2.72 (dq, J = 13.2, 8.8 Hz, 1H), 3.00 (dtd, J
13.6, 8.8, 2.8 Hz, 1H), 3.05-3.22 (m, 2H), 5.87 (t, J = 8.8 Hz, 1H),
4
6.82 (s, 1H), 6.97 (s, 1H), 7.00 (t, J = 0.8 Hz, 1H), 7.35 (t, J = 7.6 Hz,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
H), 7.44 (t, J = 8.0 Hz, 2H), 7.59 (d, J = 7.6 Hz, 2H); C{ H} NMR
13
1
(100 MHz, CDCl
3
) δ = 19.5, 20.8, 21.1, 22.2, 33.5, 58.7, 112.5,
3
.25 mmol), p-toluenesulfonyl chloride (926 mg, 4.86 mmol, 1.5
equiv), trimethylamine hydrochloride (30.9 mg, 0.32 mmol), tri-
ethylamine (900 L, 6.49 mmol, 2.0 equiv) in CH Cl (32 mL) was
stirred at room temperature for 1 h. Then, the mixture was
quenched by addition of sat. NaHCO aq. The crude product was
extracted with CH Cl , washed with brine, dried over Na SO , and
concentrated under reduced pressure. Then the crude product
was dissolved in 48.6 mL of CH CN and stirred at 75 °C in an oil
1
1
+
23.2, 128.1, 129.5, 129.8, 131.2, 131.8, 135.6, 137.0, 137.6,
–
38.3, 140.3, 169.4. HRMS (DART) calcd for C42
H
44CuN (M–Cl
4
2
2
4by) 667.2857, found 667.2849. Elemental analysis of the im-
pure product indicated that the main complex is probably
CuCl((R)-4by) and not [Cu((R)-4by) ]Cl. Anal. Calcd for
(CuCl((R)-4by)) C, 62.84; H, 5.72; N, 6.98, for
([Cu((R)-4by) ]Cl) C, 71.67; H, 6.30; N, 7.96, found
3
2
2
2
2
4
C
C
21
H
H
22ClCuN
44ClCuN
2
4
42
2
3
25.4
C, 60.43; H, 5.24, N, 6.27. []
Procedures for the Asymmetric Borylation of ,-Unsaturated
Esters
General Procedure C: (R)-4by (5 mol %, 0.017 mmol), (Bpin)
D
= +216.3 (c = 0.10, CHCl
3
).
bath. After 20 h, the mixture was cooled to room temperature,
added 48.6 mL of MeOH, and stirred at 75 °C for 10 h. After con-
centration, the residue was partitioned between Et
The organic layer was extracted with 1N HCl twice. The combined
aqueous extracts were adjusted to pH = 8 by addition of NaOH
2
O and H O.
2
2
(2.0 equiv, 0.704 mmol), t-BuOK (1.0 equiv, 0.352 mmol), and
molecular sieve 3A (300 mg/mmol) were weighed into a flask. To
this was added THF (1.76 mL). Then the mixture was stirred at
room temperature for 10 min. After cooling to –40 °C, alkyl cin-
namate 5 (0.352 mmol) and MeOH (2.0 equiv, 0.704 mmol) were
added to the mixture. The resulting mixture was stirred at –40 °C
solution and extracted with CH
were dried over Na SO , filtered, and concentrated. The residue
was purified by short silica gel column chromatography (first
CH Cl only, then MeOH) to give (rac)-1b (674 mg, 2.98 mmol,
2% yield). The enantiomerically pure (R)- and (S)-1b were ob-
Cl
2 2
three times. The organic layers
2
4
2
2
9
4b
for 3 h and quenched with NaBO
(1.76 mL). After stirring for 1.5 h at room temperature, the crude
product was extracted with EtOAc, dried over Na SO , concentrat-
3
•4H O (1.94 mmol) and water
2
tained by separation using preparative HPLC.
Procedures for the Preparation of 2-Aryl-imidazolium Salts 2
General Procedure A: A mixture of imidazole 1, diaryliodonium
2
4
tetrafluoroborate (1.5 equiv), and Cu(OAc)
2
•H
2
O (7.25 mol%) in
ed under reduced pressure, and purified by silica gel column
chromatography (EtOAc/hexane = 2/1) to give 6. The enantio-
meric excess of the product was determined by HPLC analysis
with chiral stationary phase column (Daicel Chiralcel OD-H, Chi-
ralpak AS-H, or Chiralpak AD-H).
DMF was heated to 100 ºC in an oil bath and stirred for 4 h. After
cooling to room temperature, the mixture was concentrated un-
der reduced pressure and purified by silica gel column chroma-
tography to give the desired imidazolium salt 2.
(S)-Ethyl-3-hydroxy-3-phenylpropanate ((S)-(–)-6a); Following
(
R)-2-Phenyl-5-mesityl-6,7-dihydro-5H-pyrrolo[1,2-c] imidazol-
-ium tetrafluoroborate ((R)-2by). Following the General Proce-
dure A: (R)-1b (100.0 mg, 0.442 mmol), diphenyliodonium tetra-
fluoroborate (243.8 mg, 0.663 mmol), Cu(OAc) •H O (6.4 mg,
.032 mmol), and DMF (4.1 mL) were used; purified by silica gel
column chromatography (CH Cl /hexane/MeOH = 5/5/1) to give
the title compound (151.2 mg, 0.387 mmol, 88% yield); H NMR
the General Procedure C; 75% yield (37.8 mg, 0.195 mmol); This
product was characterized by comparison of the spectroscopic
2
14a
data with those reported previously;
Daicel Chiralcel OD-H,
= 13.5 min (ma-
= –39.6 (c = 1.00,
2
2
hexane/i-PrOH = 90/10, flow rate 0.5 mL/min, t
S
0
25.0
jor), t
CHCl ).
R
= 16.4 min (minor), 79% ee. []
D
2
2
1
3
(
(
3
400 MHz, CDCl
dq, J = 13.6, 9.6 Hz, 1H), 3.10 (dtd, J = 14.0, 8.8, 2.8 Hz, 1H), 3.24-
.39 (m, 2H), 6.41 (t, J = 9.2 Hz, 1H), 6.82 (s, 1H), 6.95 (s, 1H), 7.33
3
) δ = 1.82 (s, 3H), 2.26 (s, 3H), 2.54 (s, 3H), 2.74
(S)-Isopropyl-3-hydoxy-3-phenylpropanoate ((S)-(–)-6b); Fol-
lowing the General Procedure C; 83% yield (56.5 mg, 0.271
mmol); This product was characterized by comparison of the
spectroscopic data with those reported previously;14a Daicel Chi-
13
1
(
d, J = 1.2 Hz, 1H), 7.49-7.59 (m, 5H), 8.21 (s, 1H). C{ H} NMR
(100 MHz, CDCl
3
) δ = 19.2, 20.3, 20.7, 22.8, 34.1, 60.5, 114.2,
ralpak AD-H, hexane/i-PrOH = 98/2, flow rate 0.5 ml/min, t =
R
25.4
1
1
3
22.5, 128.2, 129.0, 130.0, 130.1, 130.3, 131.6, 135.1, 135.2,
53.9 min (minor), t
= 1.00, CHCl ).
S
= 56.9 min (major), 73% ee. []
D
= –32.1 (c
–
38.2, 138.9, 140.2. HRMS (ESI) calcd for C21
H
N
23 2
(M–BF
4
)
3
24.6
(S)-Methyl-3-hydroxy-3-phenylpropanoate ((S)-(–)-6c); Follow-
ing the General Procedure C; 69% yield (18.4 mg, 0.102 mmol);
This product was characterized by comparison of the spectro-
03.1861, found 303.1857. []
Procedures for the Preparation of NHC/Cu Complexes 4
General Produce B-1 (conditions I): A mixture of 2, Ag
D
= +84.8 (c = 1.00, CHCl
3
).
2
O (2.5
14b
equiv), and molecular sieves 4A in 1,2-dichloroethane was heated
to 80 °C in an oil bath and stirred overnight in dark. After cooling
to room temperature, the resulting suspension was filtered
through Celite to remove insoluble silver salts. Then the filtrate
was concentrated under reduced pressure, dissolved in CH
under nitrogen, and added a solution of CuCl (1.2 equiv) in CH
The mixture was stirred for 1 h at room temperature, then filtered
through Celite, and purified by reprecipitation to give 4.
General Produce B-2 (conditions Ⅱ): To a mixture of CuCl (1.0
equiv), t-BuOK (1.2 equiv), and molecular sieve 4A in THF was
added a solution of 2 in THF. The resulting mixture was stirred
for 4 h at room temperature. Then, the mixture was concentrated
under reduced pressure and purified by reprecipitation to give 4.
scopic data with those reported previously ; Daicel Chiralcel
OD-H, hexane/i-PrOH = 90/10, flow rate 0.5 mL/min, t = 18.3
= –32.1 (c =
S
25.2
min (major), t
1.00, CHCl ).
R
= 28.4 min (minor), 81% ee. []
D
3
2
Cl
Cl
2
(S)-Methyl 3-hydroxy-3-(p-tolyl)propanoate ((S)-(–)-6d); Fol-
lowing the General Procedure C; 88% yield (85.1 mg, 0.438
mmol). This product was characterized by comparison of the
2
2
.
14b
spectroscopic data with those reported previously ; Daicel Chi-
ralpak AD-H, hexane/i-PrOH = 98/2, flow rate 0.8 mL/min, t =
R
25.4
47.0 min (minor), t
= 1.00, CHCl ).
S
= 48.9 min (major), 81% ee. []
D
= –31.6 (c
3
(–)-Methyl 3-hydroxy-3-(m-tolyl)propanoate ((–)-6e); Following
1
the General Procedure C; 87% yield (58.2 mg, 0.300 mmol). H
ACS Paragon Plus Environment

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The Journal of Organic Chemistry
NMR (400 MHz, CHCl
3
) δ = 2.35 (s, 3H), 2.17-2.76 (m, 2H), 3.17 (s,
(R)-4by (5 mol %, 39.0 mg, 0.0969 mmol), (Bpin) (2.0 equiv, 984
2
1
2
3
4
5
6
7
8
9
1H), 3.73 (s, 3H), 5.10 (dt, J= 9.2, 4.0 Hz, 1H) 7.09-7.26 (m, 4H).
mg, 3.86 mmol), t-BuOK (1.0 equiv, 217 mg, 1.93 mmol), molecu-
lar sieve 3A (579 mg), THF (9.7 mL), ethyl cinnamate (5a) (340
mg, 1.93 mmol), and MeOH (2.0 equiv, 156 L, 3.86 mmol) were
used.
13
1
C{ H} NMR (100 MHz, CDCl
3
) δ = 21.4, 43.1, 51.9, 70.3, 122.7,
1
26.3, 128.4, 128.5, 138.2, 142.4, 172.8. Daicel Chiralcel OD-H,
hexane/i-PrOH = 90/10, flow rate 0.5 ml/min, t = 15.4 min (ma-
25.7
jor), t = 19.6 min (minor), 84% ee. []
CHCl ). HRMS (ESI) calcd for C11
found 217.0831.
D
= –17.7 (c = 1.00,
+
3
H
14
3
O Na (M+Na ) 217.0841,
ASSOCIATED CONTENT
(
–)-Methyl 3-hydroxy-3-(o-tolyl)propanoate ((–)-6f); Following
Supporting Information
1
the General Procedure C; 76% yield (52.3 mg, 0.269 mmol). H
NMR (400 MHz, CHCl
s, 1H), 3.74 (s, 3H), 5.35 (dt, J = 9.2, 3.2 Hz, 1H), 7.13-7.25 (m,
H), 7.50 (d, J = 7.6 Hz, 1H). C{ H} NMR (100 MHz, CDCl
18.9, 41.9, 51.9, 66.9, 125.1, 126.4, 127.6, 130.4, 134.2, 140.4,
72.9. Daicel Chiralcel OD-H, hexane/i-PrOH = 90/10, flow rate
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.xxxxxxx.
3
); δ = 2.34 (s, 3H), 2.67-2.70 (m, 2H), 3.12
(
3
1
13
1
1
H, C{ H} NMR spectra of new compounds; H spectra of
known compounds; chiral HPLC analysis of compound 6
PDF).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
13
1
3
) δ =
(
1
0
[
(
.5 ml/min, t = 17.0 min (major), t = 26.3 min (minor), 75% ee.
2
5.6
AUTHOR INFORMATION
Corresponding Author
E-mail: kyoshida@faculty.chiba-u.jp
]
D
+
= –31.2 (c = 1.00, CHCl
3
). HRMS (ESI) calcd for C11
M+Na ) 217.0841, found 217.0831.
S)-Methyl 3-(3-chlorophenyl)-3-hydroxypropanoate ((S)-(–)-
6g); Following the General Procedure C; 89% yield (33.9 mg,
.147 mmol). This product was characterized by comparison of
H
14 3
O Na
(
*
0
ORCID
14b
the spectroscopic data with those reported previously ; Daicel
Chiralpak AD-H, hexane/i-PrOH = 90/10, flow rate 0.5 mL/min, t
17.6 min (minor), t
c = 1.00, CHCl ).
(–)-Methyl 3-(3,5-dimethylphenyl)-3-hydroxypropanoate ((–)-
h); Following the General Procedure C; 89% yield (30.7 mg,
.147 mmol). H NMR (400 MHz, CHCl
.73 (m, 2H), 3.12 (d, J = 3.6 Hz, 1H), 3.73 (s, 3H), 5.07 (dt, J = 9.6,
R
Kazuhiro Yoshida: 0000-0001-5130-9479
2
5.5
D
=
(
S
= 19.7 min (major), 84% ee. []
= –19.0
Notes
3
The authors declare no competing financial interest.
6
0
2
3
1
3
); δ = 2.31 (s, 6H), 2.70-
ACKNOWLEDGMENT
13
1
.6 Hz, 1H), 6.92 (s, 1H), 6.98 (s, 2H). C{ H} NMR (100 MHz,
) δ = 21.3, 43.1, 51.8, 70.3, 123.4, 129.4, 138.1, 142.4, 172.9.
This work was supported by Grant-in-Aid for Scientific Re-
search on Innovative Areas "Precisely Designed Catalysts with
Customized Scaffolding" from MEXT to K.Y. (Grant
#18H04236), Grant-in-Aid for Scientific Research (C) from
JSPS to K.Y. (Grant #18K05097), the Futaba Research Grant
Program of the Futaba Foundation to K.Y.
CDCl
3
Daicel Chiralpak AS-H, hexane/i-PrOH = 90/10, flow rate 0.5
mL/min, t = 13.8 min (minor), t = 15.7 min (major), 83% ee.
2
4.9
[
(
]
D
+
= –42.7 (c = 1.00, CHCl
3
). HRMS (ESI) calcd for C12
M+Na ) 231.0997, found 231.0986.
S)-Methyl 3-hydroxy-3-(naphthalen-2-yl)propanoate ((S)-(–)-
i); Following the General Procedure C; 79% yield (41.8 mg,
H
16 3
O Na
(
6
0.182 mmol). This product was characterized by comparison of
the spectroscopic data with those reported previously ; Daicel
Chiralpak AD-H, hexane/i-PrOH = 90/10, flow rate 0.5 mL/min, t
14c
REFERENCES
R
(
1) For recent reviews on NHCs, see: (a) Hopkinson, M. N.;
25.6
=
(
34.8 min (minor), t
c = 1.00, CHCl ).
S)-Methyl
S
= 37.4 min (major), 83% ee. []
D
= –32.7
Richter, C.; Schedler, M.; Glorius, F. “An overview of N-heterocyclic
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3
(
3-hydroxy-3-(4-trifluoromethylphenyl)propanoate
((S)-(–)-6j); Following the General Procedure C; 46% yield (19.6
mg, 0.078 mmol). This product was characterized by comparison
of the spectroscopic data with those reported previously ; Daicel
Chiralpak AD-H, hexane/i-PrOH = 90/10, flow rate 0.5 ml/min, t
19.7 min (major), t
c = 1.00, CHCl ).
(S)-Methyl 3-(4-chlorophenyl)-3-hydroxypropanoate ((S)-(–)-
k); Following the General Procedure C; 89% yield (35.1 mg,
.163 mmol). This product was characterized by comparison of
the spectroscopic data with those reported previously ; Daicel
Chiralpak AS-H, hexane/i-PrOH = 95/5, flow rate 1.0 mL/min, t
7.9 min (minor), t
= 0.40, CHCl ).
(S)-Methyl 3-hydroxy-4-methylpentanoate ((S)-(–)-6l); Follow-
ing the General Procedure C; 73% yield (49.1 mg, 0.335 mmol).
This product was characterized by comparison of the spectro-
scopic data with those reported previously ; Enantiomeric ex-
cess was determined by HPLC analysis after conversion into the
14c
S
5.4
=
(
R
= 21.0 min (minor), 85% ee. []2
D
= –6.8
3
(
2) For reviews on chiral NHCs, see: (a) Perry, M. C.; Burgess, K.
“
Chiral N-heterocyclic carbene-transition metal complexes in
6
0
asymmetric catalysis” Tetrahedron: Asymmetry 2003, 14, 951-
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Yasue, R. “Planar-Chiral Ferrocene-Based N-Heterocyclic Carbene
Ligands” Chem. Eur. J. 2018, 24, 18575-18586.
14b
R
=
25.0
1
S
= 19.8 min (major), 85% ee. []
D
= –18.5 (c
3
14b
6i
corresponding benzoate using a chiral stationary phase column
Daicel Chiralpak AD-H, hexane/i-PrOH = 95/5, flow rate 0.8
mL/min, t
R
= 8.84 min (minor), t
= –28.2 (c = 1.00, CHCl ).
Lager Scale Procedure for the Asymmetric Borylation of ,-
S
= 10.4 min (major), 68% ee.
2
5.7
[
]
D
3
Unsaturated Esters
(S)-Ethyl-3-hydroxy-3-phenylpropanate ((S)-(–)-6a); Following
the General Procedure C; 81% yield (304 mg, 1.56 mmol), 76% ee.
(
3) Zhou, Q.-L. Ed. Privileged Chiral Ligands and Catalysts;
Wiley-VCH: Weinheim, 2011.
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The Journal of Organic Chemistry
Page 6 of 6
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
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1
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0
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